That is a tough question. It really depends on load and it is more related to belt contact surface. This dictates more wrap around a small pulley under the same load as a large pulley.
If you are planning on running a 2" supercharger pulley at a high boost level it will slip. I ran about this size on a early version of my car with the slk supercharger. I was only running 6lbs of boost though and the rpms were to high for the supercharger. I currently run a 2.75" pulley and wrap almost as much as possible. I made the mounting capable of adjusting the idler pulley to change the wrap and the belt length.
Back to the rpm. You really have to consider this. With a 6" crank pulley and a 2" drive pulley you are at 3:1. So at 6000 rpm you are driving the 2" pulley at 18000 rpm. Most superchargers do not like this or anything else.