The max tire size you can get on a Rockhopper is a 2.3 but it can vary with different brands. On my Rockhopper I am running a 2.3 purgatory in front and a 2.1 captain on the rear. On my 29er Rockhopper I am using 2.1 captains front and back.
Hope that helps out.
If you have anymore Rockhopper questions or comments I'll do my best to respond in a day or two.