CoreRPG is a Fantasy Grounds ruleset, just like 5E is a ruleset. You can create a new campaign using the CoreRPG rules - it is a lot lighter than the 5E ruleset. Testing in CoreRPG reduces a lot of the FG code and so can help identify if there is an issue elsewhere. v3.1.7 is the current "live" release of Fantasy Grounds (not the test release).
Go for the smallest you can live with.
As I mentioned in post #26, the issue is running out of the limited memory that the 32-bit Fantasy Grounds application can use. To the best of my knowledge there is no equation available that accurately estimates the amount of memory that a specific image will use. The more pixels an image has the more Fantasy Grounds memory it will take when it is opened in FG. You might be able to open two maps that are 10,000 x 10,000 pixels in size (probably not with the layers extension), but I'd be surprised if you could open three or four of them at once and still use Fantasy Grounds reliably! As Fantasy Grounds frequently has multiple windows open (all taking resources) it is best to make sure that each individual map/image is as optimized as it can be. Sorry, I know this doesn't give you a definitive answer to your question.
UPDATE: I've just done some testing with a few really large image files (10,000 x 10,000) and I can only open three of them before FG starts to behave weirdly and give very similar errors to those reported earlier in this thread.

