i usally use 400x130 but thats just me talkin but thats what the Guys at http://www.silverstorm-design.net/forums say to if you dont know about ssd then check it out they make sigs for people.
Right now mostly 350x150 but awhile ago I was using 400x100 then 400x120 and right now 450x200 yes its giant but you can get full effects of the brushes. Those huge sigs are just for getting my brush skills good