It depends on what the head pressure is...How much flow you want out of it. What your overflow is like. If you have a standard 1" overflow then you don't want to go more than 600gph, because that's about the max rate the overflow can drain. This can be upped if you have multiple overflow pipes, or 1.5" ones.
I did fine with a mag 5 on my 55 on a softie, lps tank. But I had to put 2 maxi-jet 12 powerheads in the tank for added water movement.
I'd say probably a mag5 or mag7.