In my experience, bigger steels generate more sparks. Probably because the size allows more contact area between the stiker and the firesteel. You can get more sparks from a smaller rod by striking it (actually a scraping motion) with the toothed side of a hacksaw blade. This really produces the sparks, but eats up your firesteel faster. I imagine a hacksaw blade on one of those large 1/3 or 1/2 inch diameter firesteels would light things up like the 4th of July!

AFAIK, firesteels are all the same material, just different in diameter (and maybe protective coating).