I just screengrabbed mine and cropped the image so it was just the signature. then all I had to do was look at the canvas size or image properties to see what size it was
mine's 591x400something.
I was wondering when we'd get signature limits. it was just a matter of time for some people to get enough images to fit the whole screen
it might be easier to do a code that puts scrollbars if you signature is too big, so if someone wants to see the whole thing, all they have to do is use those scrollbars. that's what I do.
weird though that I have the same size for signature limitations (well, after that point it does scrollbars) on my forums