Oh, wait. You mean your profile? OKAY! xD
When you modify your profile and scroll down, there will be an area thingy that says about your signature, and a little bar underneath that says 400. So you write on the thingy, though the 400 will go down one each, though that is just a limit to how many letters you are able to type in your signature. =D Whisper me if you need any more help, or if I sound like the idiot person who confuses everyone like I almost can do all the time!!!