He is restricted by weight and size due to the geometry and current weight of the machine. Ideally you should go for the biggest Diameter you can but looking at Rango its not totally necessary if you have out right speed. I cant remember the graph shape but you can gain back your RMI in speed but doing it in the mass of the weapon is ^2 in terms of efficiency... I may have that wrong.

A 1.5-1.8Kg Rango style Drisk doing 12'5000 would pack a massive punch either way!