It is possible to set this higher, indeed. I don't have much objections to increasing the limit a bit, but you have to keep in mind that it is close to impossible to check PM's for us and that malicious users could in theory abuse large PM's to fill the database with rubbish, lowering speed and database space left. I do not think this would be a big problem, though. If the other admins agree, I'd suggest increasing it to about 7500/10k characters (max character count for posts is 10k).