There is now a CSS-based automatic size limit on signatures. All signatures higher than 125 pixels (this was voted on by the staff and elite members) or wider than your browser window will be cut off. You can see an example in my current sig.
Profiles will still show the entire signatures.
Also, there's a new signature separator as I posted in a T&T thread a few days ago.
You can post comments or problems in this thread. I'd be interested to know if everything works right in Mozilla... I've only tested Opera 7 and IE 6.