there was nothing mentioned about them in the forum rules.
what are the limits
i enjoy having my sig there
RTM
You can't have images in there, I don't think they've set up the limits to do with how many lines etc yet.
Obviously, don't be ridiculous and have 24 pages worth of text in there, keep it to a reasonable size. Normally, the rules are a few lines of text, as this doesn't affect the viewing of the forum too much.