diff --git a/.github/workflows/size.yml b/.github/workflows/size.yml
index 825fc219c..168923d9b 100644
--- a/.github/workflows/size.yml
+++ b/.github/workflows/size.yml
@@ -28,7 +28,7 @@
 
 name: Size
 
-on: [pull_request]
+on: [push, pull_request]
 
 jobs:
 
@@ -44,9 +44,15 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
+    - name: Bootstrap
+      if: "github.event_name == 'push'"
+      run: |
+        python3 -m pip install --upgrade setuptools wheel
+        python3 -m pip install mdv
     - name: Run
       env:
         OT_BASE_BRANCH: "${{ github.base_ref }}"
         SIZE_REPORT_URL: "https://openthread-size-report.glitch.me/size-report/1354027"
       run: |
+        export PATH=$PATH:$HOME/.local/bin
         ./script/check-size
diff --git a/script/bootstrap b/script/bootstrap
index 7d9938290..24e37c2f6 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -31,27 +31,23 @@
 #      example compilation and programming.
 #
 
-die()
-{
-    echo " *** ERROR: " $*
-    exit 1
-}
+set -e
 
 install_packages_apt()
 {
     # apt update and install dependencies
-    sudo apt-get update || die
-    sudo apt-get --no-install-recommends install -y automake g++ libtool lsb-release make cmake ninja-build || die
+    sudo apt-get update
+    sudo apt-get --no-install-recommends install -y automake g++ libtool lsb-release make cmake ninja-build
 
     PLATFORM=$(lsb_release -is)
 
-    if [ $PLATFORM = "Raspbian" ]; then
-        sudo apt-get --no-install-recommends install -y binutils-arm-none-eabi gcc-arm-none-eabi gdb-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib || die
+    if [ "$PLATFORM" = "Raspbian" ]; then
+        sudo apt-get --no-install-recommends install -y binutils-arm-none-eabi gcc-arm-none-eabi gdb-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib
     else
         # add gcc-arm-embedded ppa
-        sudo add-apt-repository ppa:team-gcc-arm-embedded/ppa -y || die
-        sudo apt-get update || die
-        sudo apt-get --no-install-recommends install -y gcc-arm-embedded || die
+        sudo add-apt-repository ppa:team-gcc-arm-embedded/ppa -y
+        sudo apt-get update
+        sudo apt-get --no-install-recommends install -y gcc-arm-embedded
     fi
 
     # add clang-format for pretty
@@ -59,6 +55,9 @@ install_packages_apt()
 
     # add yapf for pretty
     python3 -m pip install yapf || echo 'Failed to install python code formatter yapf. Install it manually if you need.'
+
+    # add mdv for local size report
+    python3 -m pip install mdv || echo 'Failed to install markdown render. Install it manually if you need.'
 }
 
 install_packages_opkg()
@@ -81,14 +80,14 @@ install_packages_brew()
     brew install arm-none-eabi-gcc
 
     # check for gcc for simulation
-    if ! which gcc; then
+    if ! command -v gcc; then
         echo 'warning: clang/gcc needed for simulation'
         echo 'warning: please install Command Line Tools from https://developer.apple.com/download/more/'
     fi
 
     # add clang-format for pretty
     CLANG_FORMAT_VERSION="clang-format version 6.0"
-    which clang-format-6.0 || (which clang-format && (clang-format --version | grep -q "${CLANG_FORMAT_VERSION}")) || {
+    command -v clang-format-6.0 || (command -v clang-format && (clang-format --version | grep -q "${CLANG_FORMAT_VERSION}")) || {
         brew install llvm@6
         sudo ln -s "$(brew --prefix llvm@6)/bin/clang-format" /usr/local/bin/clang-format-6.0
     }
@@ -105,13 +104,13 @@ install_packages_source()
 install_packages()
 {
     PM=source
-    if which apt-get; then
+    if command -v apt-get; then
         PM=apt
-    elif which rpm; then
+    elif command -v rpm; then
         PM=rpm
-    elif which opkg; then
+    elif command -v opkg; then
         PM=opkg
-    elif which brew; then
+    elif command -v brew; then
         PM=brew
     fi
     install_packages_$PM
diff --git a/script/check-size b/script/check-size
index a205a8174..32fe10e21 100755
--- a/script/check-size
+++ b/script/check-size
@@ -27,7 +27,12 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
-set -euxo pipefail
+set -euo pipefail
+
+readonly OT_TMP_DIR=/tmp/ot-size-report
+readonly OT_SHA_NEW=${GITHUB_SHA:-$(git rev-parse HEAD)}
+readonly OT_SHA_OLD="$(git cat-file -p "${OT_SHA_NEW}" | grep 'parent ' | head -n1 | cut -d' ' -f2)"
+readonly OT_REPORT_FILE=/tmp/size_report
 
 setup_arm_gcc_7()
 {
@@ -43,19 +48,58 @@ setup_arm_gcc_7()
     arm-none-eabi-gcc --version
 }
 
+setup()
+{
+    setup_arm_gcc_7
+}
+
+markdown_init()
+{
+    echo '|  name  |  branch  |  text  | data  | bss  | total |'
+    echo '| :----: | :------: | -----: | ----: | ---: | ----: |'
+}
+
+markdown_size()
+{
+    local name
+    name=$(basename "$1")
+
+    read -r -a size_old <<< "$(size "$1" | awk '{text+=$1} {bss+=$2} {data+=$3} {total+=$4} END {printf "%d %d %d %d", text, bss, data, total}')"
+    read -r -a size_new <<< "$(size "$2" | awk '{text+=$1} {bss+=$2} {data+=$3} {total+=$4} END {printf "%d %d %d %d", text, bss, data, total}')"
+
+    local -a size_diff
+
+    for i in 0 1 2 3; do
+        size_diff[$i]="$((size_new["$i"] - size_old["$i"]))"
+        if [[ "${size_diff["$i"]}" != 0 ]]; then
+            size_diff["$i"]=$(printf '%+d' "${size_diff["$i"]}")
+        fi
+    done
+
+    echo "| ${name} | -${OT_SHA_OLD:0:7} | ${size_old[0]} | ${size_old[1]} | ${size_old[2]} | ${size_old[3]} |"
+    echo "|  | +${OT_SHA_NEW:0:7} | ${size_new[0]} | ${size_new[1]} | ${size_new[2]} | ${size_new[3]} |"
+    echo "|  | +/- | ${size_diff[0]} | ${size_diff[1]} | ${size_diff[2]} | ${size_diff[3]} |"
+}
+
+markdown()
+{
+    case "$1" in
+        init)
+            shift
+            markdown_init "$@" > "${OT_REPORT_FILE}"
+            ;;
+        size)
+            shift
+            markdown_size "$@" >> "${OT_REPORT_FILE}"
+            ;;
+        post)
+            mdv "${OT_REPORT_FILE}"
+            ;;
+    esac
+}
+
 size_nrf52840()
 {
-    mkdir ../output
-
-    # The first parent should be the merge base
-    MERGE_BASE_SHA="$(git cat-file -p "${GITHUB_SHA}" | grep 'parent ' | head -n1 | cut -d' ' -f2)"
-
-    # Fetch base commit in case it is not present
-    git fetch --depth 1 --no-recurse-submodules origin "${MERGE_BASE_SHA}"
-
-    export OT_MERGE_BASE_SHA
-
-    # pull request
     local flags=(
         "BORDER_AGENT=1"
         "BORDER_ROUTER=1"
@@ -84,57 +128,71 @@ size_nrf52840()
         "UDP_FORWARD=1"
     )
 
-    git checkout -- .
-    git clean -xfd
-    ./bootstrap
-    make -f examples/Makefile-nrf52840 "${flags[@]}"
-    mv output/nrf52840 ../output/nrf52840-b
+    rm -rf "${OT_TMP_DIR}"
 
-    git checkout -f "${MERGE_BASE_SHA}"
-    git submodule update --init
+    # new commit
+    mkdir -p "${OT_TMP_DIR}/b"
+    git archive "${OT_SHA_NEW}" | tar x -C "${OT_TMP_DIR}/b"
 
-    # base branch
-    git checkout -- .
-    git clean -xfd
-    ./bootstrap
-    make -f examples/Makefile-nrf52840 "${flags[@]}"
-    mv output/nrf52840 ../output/nrf52840-a
+    (cd "${OT_TMP_DIR}/b" \
+        && ./bootstrap \
+        && make -f examples/Makefile-nrf52840 "${flags[@]}")
 
-    curl -s "${SIZE_REPORT_URL}/bash" > size-report
-    chmod a+x size-report
+    # old commit
+    if [[ "${GITHUB_ACTIONS+x}" ]]; then
+        git fetch --depth 1 --no-recurse-submodules origin "${OT_SHA_OLD}"
+    fi
 
-    ./size-report init OpenThread
+    mkdir -p "${OT_TMP_DIR}/a"
+    git archive "${OT_SHA_OLD}" | tar x -C "${OT_TMP_DIR}/a"
+    (cd "${OT_TMP_DIR}/a" \
+        && ./bootstrap \
+        && make -f examples/Makefile-nrf52840 "${flags[@]}")
 
-    ./size-report size ../output/nrf52840-a/bin/ot-cli-ftd ../output/nrf52840-b/bin/ot-cli-ftd
-    ./size-report size ../output/nrf52840-a/bin/ot-cli-mtd ../output/nrf52840-b/bin/ot-cli-mtd
-    ./size-report size ../output/nrf52840-a/bin/ot-ncp-ftd ../output/nrf52840-b/bin/ot-ncp-ftd
-    ./size-report size ../output/nrf52840-a/bin/ot-ncp-mtd ../output/nrf52840-b/bin/ot-ncp-mtd
-    ./size-report size ../output/nrf52840-a/bin/ot-rcp     ../output/nrf52840-b/bin/ot-rcp
+    local reporter
 
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-cli-ftd.a ../output/nrf52840-b/lib/libopenthread-cli-ftd.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-cli-mtd.a ../output/nrf52840-b/lib/libopenthread-cli-mtd.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-ftd.a     ../output/nrf52840-b/lib/libopenthread-ftd.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-mtd.a     ../output/nrf52840-b/lib/libopenthread-mtd.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-ncp-ftd.a ../output/nrf52840-b/lib/libopenthread-ncp-ftd.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-ncp-mtd.a ../output/nrf52840-b/lib/libopenthread-ncp-mtd.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-rcp.a     ../output/nrf52840-b/lib/libopenthread-rcp.a
-    ./size-report size ../output/nrf52840-a/lib/libopenthread-radio.a   ../output/nrf52840-b/lib/libopenthread-radio.a
+    # not on GitHub Actions or not a pull request event
+    if [[ ! "${GITHUB_ACTIONS+x}" || ! "${GITHUB_REF-}" =~ ^refs/pull/[0-9]+/merge ]]; then
+        reporter=markdown
+    else
+        reporter=./size-report
+        curl -s "${SIZE_REPORT_URL}/bash" > size-report
+        chmod a+x size-report
+        export OT_SHA_NEW OT_SHA_OLD
+    fi
 
-    ./size-report post
+    "${reporter}" init OpenThread
+
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/bin/ot-cli-ftd "${OT_TMP_DIR}"/b/output/nrf52840/bin/ot-cli-ftd
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/bin/ot-cli-mtd "${OT_TMP_DIR}"/b/output/nrf52840/bin/ot-cli-mtd
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/bin/ot-ncp-ftd "${OT_TMP_DIR}"/b/output/nrf52840/bin/ot-ncp-ftd
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/bin/ot-ncp-mtd "${OT_TMP_DIR}"/b/output/nrf52840/bin/ot-ncp-mtd
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/bin/ot-rcp     "${OT_TMP_DIR}"/b/output/nrf52840/bin/ot-rcp
+
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-cli-ftd.a "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-cli-ftd.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-cli-mtd.a "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-cli-mtd.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-ftd.a     "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-ftd.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-mtd.a     "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-mtd.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-ncp-ftd.a "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-ncp-ftd.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-ncp-mtd.a "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-ncp-mtd.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-rcp.a     "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-rcp.a
+    "${reporter}" size "${OT_TMP_DIR}"/a/output/nrf52840/lib/libopenthread-radio.a   "${OT_TMP_DIR}"/b/output/nrf52840/lib/libopenthread-radio.a
+
+    "${reporter}" post
 }
 
 main()
 {
     if [[ $# == 0 ]]; then
-        setup_arm_gcc_7
+        setup
         size_nrf52840
     elif [[ "$1" == setup ]]; then
-        setup_arm_gcc_7
+        setup
     elif [[ "$1" == nrf52840 ]]; then
         size_nrf52840
     else
         echo "USAGE: $0 [setup|nrf52840]"
-        exit 1
+        exit 128
     fi
 }