The critical dimension is the distance from the saddle or seat post to the front handlebar. This affects your riding posture and how far forward you need to reach to access the shift levers and the brakes. This dimension varies from one bike to the next regardless of the "size" of the bike. I would go to the nearest bike shop and get on some bikes and see which type and size are the most comfortable. The amount of front fork rake also varies and can make a "pro" or "expert" level bike seem overly sensitive and squirrelly for a novice and this will be noticed in particular on downhill runs.
Second key dimension is the standover height so you can stop and not have to worry about damage to the crotch area.