It should be alright. We've got people claiming essentially all of Eastern or Western Europe. As long as it makes sense and is within the factions capability to hold. Japan did hold most of the pacific for a time.
However if there's a conflict of interest(You and Lax both want the same land) you guys should discuss that. If it's because you have a large neighbor, then it depends on the coalitions you're all a part of. No matter how large someone is, they can't really hold out against a lot of factions attacking them at once(If it's purely numbers)