If all of the Mono add-ons are inside a single folder, we could just deduct the size of that folder.
all is under .mono, yes, well separated :)
And that folder is exactly 4.3MB? I wonder if it's the same size on Mac/Win/Linux?
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.
And so, it shall be. ;-)
Mon devs simply subtract the size of their 'mono' folder.