diff --git a/tools/idf_size.py b/tools/idf_size.py
index 80c968239a..a3cb75c7da 100755
--- a/tools/idf_size.py
+++ b/tools/idf_size.py
@@ -13,8 +13,13 @@ if __name__ == '__main__':
     # Here the argparse is used only to "peek" into arguments if
     # legacy version is requested or if old json format is specified.
     # In these two cases the esp_idf_size legacy version is spawned.
-    parser = argparse.ArgumentParser(exit_on_error=False, add_help=False)
-    parser.add_argument('--format')
+    parser = argparse.ArgumentParser(add_help=False)
+    # Make the --format arg optional, so this argparse instance does not
+    # fail with an error and the proper underlying help is displayed.
+    # Note that exit_on_error is supported from python3.9, so this is
+    # a workaround how to make sure that the args parsing doesn't fail here if idf_size.py
+    # is invoked e.g. without specifying the format, like "idf_size.py --format".
+    parser.add_argument('--format', nargs='?')
     parser.add_argument('-l', '--legacy', action='store_true', default=os.environ.get('ESP_IDF_SIZE_LEGACY', '0') == '1')
 
     # The sys.argv is parsed with "exit_on_error", but the argparse.ArgumentError
@@ -34,7 +39,9 @@ if __name__ == '__main__':
             os.environ['ESP_IDF_SIZE_NG'] = '1'
             if not rest or '-h' in rest or '--help' in rest:
                 print(('Note: legacy esp_idf_size version can be invoked by specifying the -l/--legacy '
-                       'option or by setting the ESP_IDF_SIZE_LEGACY environment variable.'))
+                       'option or by setting the ESP_IDF_SIZE_LEGACY environment variable. Additionally, the '
+                       'legacy version is automatically employed when the JSON format is specified for '
+                       'compatibility with previous versions.'))
 
     if args.format is not None:
         rest = ['--format', args.format] + rest