The SSG forums system currently limits the number of characters allowed to users for their signatures, but not how they are used. Since last year, the SSG forums have had a maximum signature policy:
Signatures must be limited to 5 lines maximum including line breaks and wrapping, and signatures may not use larger than size 4 text.
This is to avoid cluttering thread pages with oversized signatures. However, there has been some confusion as to how much width is accomodated before wrapping because some browsers and desktops display different widths. So, attached to this post is a graphic template which shows the maximum size allowed for signatures.
Because the graphic's full size remains unchanged at any desktop setting, if your signature looks like it'll go wider than the graphic, that means on some systems it will wrap down to the next line and your signature will be 1 line longer than what you are seeing, which is not allowed and could get your signature altered or deleted without warning.
Users who find that even these signature limitations leave thread pages too cluttered can turn off viewing signatures entirely via the User Control Panel.