Well technically you can take a small sig and have a render filling up most of the canvas and look like a novice. Bigger tags are harder to fill up oftentimes. Then again cramming stuff into a small sig and making it look good is also another feat. 400 x 150 seems to be the preferred size, and I think it's a good one - if you work with too small a size and you're using a render or stock, you have to shrink your render too much, it's going to decrease a lot in quality. You want to have ample space to work with so that your sig isn't just a render - but it also has a real nice bg and nice effects. It really depends on what you're doing though - if you're making a sig that advertises a site then Id say make it smaller.