yep, a 110 will fit just fine and do the job. some 120s will fit without rubbing, some will rub, just have to try em and see. I used to run 120s on the back of my airheads, but grew tired of struggling to get the wheel back on, so once I switched back to 110, I never looked back. bike handles just fine, and chances are you will never need more tire for street duty. if you were pushing out 90 hp, then I'd go bigger, but the stock 70hp(on a good day with the wind blowing just right and the proper humidity!) will be harnessed just fine with the 110 size tire.
don't fall for the big back tire syndrome, just fit the proper tire and enjoy the ride.