unless Xinapse or George change the limit it's going to be provisionally 250px(h) x600px(w). I think it's more than enough room for you all. There is also a warning in place for large pics as of today.
Without pointing out, some members fall well outside this marker and therefore I'll give a week to correct signatures, so this new rule will commence next Sunday. But this is your heads up :)
To check your signature size right-click the image and image info, that's in Firefox anyway.