If your on a budget, you could start at 61mm, however, in racing, more mass means slower accelleration. Many people are starting at 59mm or less. Look around here for roll out numbers based on your motor, then go to one of the gearing websites to figure out what ratio it is based on your choices. (hint, 64 pitch gears are more efficent at transferring power than 48, I would upgrade when funds allow)...
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.