If I'm being honest, I don't understand why anybody would want to use maps that size. 256x256 is more than enough. Getting bigger than that, they aren't fun to create/manage and they aren't fun to traverse for players.
Forgive me if this sounds rude, but that's beside the point.
Thing is, I need to make a big map of at least that size (I pan on adding stuff like ships and other transportation to reduce the aggravation of traveling), and the fact trying to make one when it's possible that constantly results in non stop crashes is aggravating.
I got to the point I can LOAD a mostly blank 500 x 500 map just fine, but I tried using the fill command as a test = Instant Crash.
Even trying to edit a map that size can result in crashes, often with reports of unhandled exceptions, which seems to indicate the program has horrible stability that needs addressed, and I checked, in Task Manager, it doesn't use THAT MUCH resources, so the program has bad stability, not that it's a bad idea IMO.