It's not like anyone has, but it breaks the page when you put an image larger than 700x125 in your signature....

So if there's a way to limit people's signatures to images of that size or smaller, it might help in the future. For a moment mine was 3300x900 (thought it would shrink), and then when I looked at a thread, it nearly formed a black hole.

Sadly, there is no option to do so. The closest you could get to controlling this is to manually remove the signatures of users who breach the so called "limit".