If all of the Mono add-ons are inside a single folder, we could just deduct the size of that folder.
Instead of measuring exact size of .mono, I would just subtract .mono's size in bytes from the whole size at the end. If it's less then 2MB, we're good to go. I would do this way, because .mono's content may vary depending on the version of Godot.
So when I'll submit my project, I'll provide a size information with and without .mono. One may validate it.