Originally Posted by
cny-bikeman
...I would think an 11/28 or 12/30 would be fine, but if you only race flat crits you could go tighter. Again, it depends on your fitness level.
...or this, just start out with 12-30 and see how it goes. If you really intend to race on any sort of regular basis, you will be buying plenty of cassettes in the coming years, and you can also have a selection for races on different terrain or conditions. They're easy to change out, just buy a chain whip and the proper lock ring tool and a wrench to turn it.
Make certain you stay with compatible stuff. Shimano is pretty good about compatibility/interchangeability.