Wow. You guys have some rocket fast internets.
I limit images to under 1 Mb. As soon as images get bigger than that, with Fantasy Grounds anyway, I find that I start having lag issues. Now, to get around that, I can stitch together several smaller images - so, effectively my map size is unlimited so long as I slice it up into small enough chunks. Bit of a PITA, but, I have used some HONKING large images for some adventures.
Granted, the fact that I'm in Japan and all my players are in the States or Canada might have a lot to do with that. And I can go over 1 Mb, but, I find that that seems to be the rough limit before things start getting laggy.
Then again, my computer was bought in 2018 and it wasn't exactly top of the line then, so, yeah... Waiting until next year when Windows 10 goes kaput and I'll get myself a new machine.
I'd use my new laptop but, it's such a PITA hooking up extra monitors and I'd hate to try to run Fantasy Grounds on this 15 inch screen. Yikes.