diff --git a/.gitignore b/.gitignore
index ab667585..4df6229a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -32,3 +32,4 @@ MANIFEST
 /build
 /cmake/
 .cache/
+sosize-*.txt
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 7470c5d9..0bc0cd08 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -89,3 +89,8 @@ endif()
 # A single command to compile and run the tests
 add_custom_target(pytest COMMAND ${PYTHON_EXECUTABLE} -m pytest -rws ${PYBIND11_PYTEST_FILES}
                   DEPENDS pybind11_tests WORKING_DIRECTORY ${testdir})
+
+# And another to show the .so size and, if a previous size, compare it:
+add_custom_command(TARGET pybind11_tests POST_BUILD
+    COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_SOURCE_DIR}/tools/libsize.py
+        $<TARGET_FILE:pybind11_tests> ${CMAKE_CURRENT_BINARY_DIR}/sosize-$<TARGET_FILE_NAME:pybind11_tests>.txt)
diff --git a/tools/libsize.py b/tools/libsize.py
new file mode 100644
index 00000000..70b811d4
--- /dev/null
+++ b/tools/libsize.py
@@ -0,0 +1,38 @@
+from __future__ import print_function
+import os
+import sys
+
+# Internal build script for generating debugging test .so size.
+# Usage:
+#     python libsize.py file.so save.txt -- displays the size of file.so and, if save.txt exists, compares it to the
+#                                           size in it, then overwrites save.txt with the new size for future runs.
+
+if len(sys.argv) != 3:
+    sys.exit("Invalid arguments: usage: python libsize.py file.so save.txt")
+
+lib = sys.argv[1]
+save = sys.argv[2]
+
+if not os.path.exists(lib):
+    sys.exit("Error: requested file ({}) does not exist".format(lib))
+
+libsize = os.path.getsize(lib)
+
+print("------", os.path.basename(lib), "file size:", libsize, end='')
+
+if os.path.exists(save):
+    with open(save) as sf:
+        oldsize = int(sf.readline())
+
+    if oldsize > 0:
+        change = libsize - oldsize
+        if change == 0:
+            print(" (no change)")
+        else:
+            print(" (change of {:+} bytes = {:+.2%})".format(change, change / oldsize))
+else:
+    print()
+
+with open(save, 'w') as sf:
+    sf.write(str(libsize))
+