That's not true. Forums I go to don't use 400 x 150 or 350 x 150. I've been to forums with 500 x 100, (with 80 x 80 avatar), 600 x 200, 550 x 150, and anything (multiple images, no restriction).

The sig sizes are fine as it is.. more importantly, people don't really pay attention to it after awhile.