donno... as a beginner... i go for 350~400*100~150 mostly by watchin u guyz... 1stly i used 2 make huge ones... like wallpapers as my sigs
as monitor resolution / size grow ( due to cheaper costs , etc) people will start to use larger px. I remember when the majority of people used 350x 100, like a year ago. Seems like 400*100 is the new trend