I find this quite a hard thing to judge, at the end of the day it would depend on what sort of forum it is. Some forums need big signature spaces, but I prefer smaller ones. Some sigs can really get in the way when you are trying to read the forum post.
Honestly, I see no need for a limit. If I add one, a couple people will just abuse it, and I really have no way of judging the appropriate size unless I check the size of each image/text link and then add it up. That can be painfully tedious and boring, and nobody wants that At the end of the day, I am fine with members having signatures, and if I see one that has too many images or something like that, I'll let them know. Otherwise, I just let bygones be bygones