It should be raised slightly for non-spoilered signatures, but within spoilers it seems to be a "trial and error thing" - it should stop caring about character limit, but sometimes it won't. I spent a could hours last night trying to fix it, somehow TK helped me fix it, though I have no idea how it was fixed.
Check my signature (in my profile if it doesn't show fully here) which is ~ 2700 characters long.