2 questions that come to my mind:
If you just limit the amount of characters a user is allowed to post, what is someone posts 1-2 characters per line and thus creates a huge signature area below every post?
What about images in signatures? I guess if someone wants to post a small image, but links to the graphic, the link can be a very long URL, which would probably exceed a set character limit while the small image itself would be perfectly fine in the signature.