As a matter of fact, we've updated the Forum Rules to allow 300 displayable characters in signatures. The limit was previously 200. However, signatures should never be unnecessarily long or tall, since that can be an annoyance to other users. The rules about annoying or distracting signatures overrides the rules about signature length.
Other tips:
Short signatures are more likely to be read by other users, while long signatures are more likely to be ignored.
If users have a lot to say, we encourage them to fill in the About You box on their Account Details page, which others can see on their profile page, along with their Location, Website, Occupation, or other information that they choose to share.