You got it, it's 650 pixels by 200 pixels.
This new rule has been decided by members of Stoptazmo and not by admins/moderators. See this thread.
You have till Monday, May 15th 2006 at 09 00 EST to make sure your signature (images+text) does not exceed the size decided. If you are seen using a bigger signature, a two week ban shall be imposed.