Using this option, you can't change actual image size. This option is to use (if needed) after image (size) has been changed (resized). For example, you have some JPG images, which is missing ExifImageWidth and ExifImageWidth tag, or values inside those two tags are wrong. Applying this menu option, those two tags will be automatically filled with correct (actual) image size values. I see no reason to manually enter those two values (as we want them to be correct).
That is, changing those two tags doesn't change image size -they just contain info about image size. For resizing, some image editing tool is needed.