Well if you would have read this topic: Changes from Senv4 -> Senv5
it would have said this about signatures:
Signatures will have a new maximum of 400 pixels wide, 150 pixels high. This may change depending on any changes I make in the future, but right now, it pretty much seems like it. No matter what though, signatures will not be 600 x 200 simply because they are too big for 800x600 resolution users.