For my discussion board I do not currently have any signature enabled since I feel like they take awhile a LOT and only really serve to advertise various things (spam related or even legit related).
I always say common sense. A huge 500 vertical pixel signature isn't common sense, and that shouldn't need to be said on most forums. As long as the page isn't being stretched too much, it doesn't really matter to me.
No, I have signatures completely disabled. I might reconsider allowing them if no links are added, but I don't see them really adding all that much to the conversation in the first place.
I don't allow anything bigger than 500 x 250 in their signature. Anything bigger and I think it makes it cumbersome to scroll through the page to read everyone's posts because of the large signatures.
Putting a specific pixel limit or line limit on signature spaces is perhaps the most anal thing to do on a forum. 99% of people have a reasonable signature and there's no reason to go around measuring the pixel space or counting the lines for a violation. For the 1% that goes over board and has an obnoxious amount of stuff in their signature, you can deal with them.
People that enforce signature rules are just looking for a reason to use their power and, in most cases, abuse it.
There's not a set size but there are limits on the amounts of pictures, colored text, and links you can have just to keep signatures from interfering with the main purpose, the posts/threads.
I currently have no limit and probably wont. I personally prefer small signatures but to each their own. If people start making like 800x800 signatures I might change my mind about it though