[Ballbarian]

I think that I may have found the limit.
I created a test map and the last line that appears to have been read by Dom3 is line number 10696.

Gandalf's MegaMap is 9152 lines long.
A SemiRandomized MegaMap reached 46973 lines!
Note that these figures all include comment lines and empty lines which are likely ignored by Dom3, but no guarantees on that.

More testing will be required before I can determine if stripping comment lines and empty lines will help or not.

I typically play on 175 province maps which only reach to about 4000 to 5000 lines est. So it is probably safe to assume that you can run a 350 province map without issue?