Assuming you wanted somewhere between 3x (lower end) - 10x (really high) turnover, that is 300gph-1000gph. I think you could almost squeeze 1000gph through a single 1/2" pipe if you really wanted, so technically speaking either size is fine. Realistically though if you want 10x turnover I'd suggest 3/4".
However assuming you stay within the "normal" range of tank turnover, 3-5x, that is a maximum of 500gph divided by 2 returns = 250gph per return. That could easily be handled by 1/2" with tons of room to spare. Technically you'll have more flow with 3/4" since there will be less head pressure, but otherwise either will work fine. There are loc-line fittings for both 1/2" and 3/4" if you choose to use that for aiming your return flow.
Personally I use dual 3/4" returns on my 120, though I have a very low turnover tank so I could easily have 1/2".
