You have to include all the variable to get meaninful info.
For 2" sch. 40 PVC, a flow of 50 GPM would see a pressure loss over a 100 foot length, of 1.73 PSI, and the water velocity would be 4.77 feet per minute. Both of these are reasonable numbers, so if you needed 50 GPM through 100 feet of pipe, 2" would be an appropriate size.
Tell us what your goal is. Perhaps we can provide further information.