As far as I know there isn't a limit in the width, it is limited by the desktop width of the user who's looking at your signature. The height is limited to 400 pixel. Or 380, but I'm almost sure it were 400.
So it's basicly what I said in my first post: The height is limited at 400 Pixel. If it's longer then it gets cut of. The width depends on the screen and desktop settings of the person who looks at the signature.