The height and other measurements are all based on the worst case scenario for all aircraft so to speak. Leg length is important since if your legs are too long, as Zoomie said, you'll be a whole lot shorter if you eject. On the flip side, if your legs are too short you won't reach the pedals in a chopper. The pedals in the Harvard move quite a bit and I never had a problem reaching them, the Jet Ranger for basic helo school is a little different case, the pedals don't move as much and I had to use a cushion behind me to push me forward enough to reach them fully. Of course there are limits to how many cushions you can use, 1 being the max. Sitting height is the same sort of thing, too tall = bad for ejection seats and Jet Rangers, too short is bad for ejection seats (your head won't rest on the head rest, which is very important) and you'll have trouble seeing out of the bigger airplanes. Even your reach is important, if you've got the inertia reel locked on your seat you won't be able to reach certain switches. So, being short isn't necessarily a problem flying jets since the seat and pedals move, but flying helicopters it's a problem since the seats don't move and the pedals are rather limited in the range of motion. They only hire people that can do all jobs, it gives them better flexibility in sending you to the different communities after you finish Moose Jaw.
As for the CT156 Harvard II, it's got a Martin Baker Mk 16 ejection seat, the same one that's in the CT155 Hawk and will be in the Joint Strike Fighter.