Racin,
I currently have a 17.5BL with a 2-cell LiPo in the car. It is ballistically fast with that getup. In fact, on my LHS carpet track, I never went more than half throttle and it was still a bullet. I tested it also with a 21.5BL motor and it was still pretty quick getting around the track. Both test were done with the stock gearing, of course. I was thinking about starting the class with probably 21.5 if running 2-cell or 17.5 if running 1-cell. If it was up to me, it would be 17.5 with 2-cell. To me, the extra length of the car helps to keep it more stable at higher speeds, especially in corners.