The numbers on a bike don't mean much. Two identical bikes will be called a "19" by one company and a "21" by another.
When you stand flat-footed over the bike, with your belt buckle up against the back of the stem, the top tube of the bike should NOT be pressing painfully into your crotch...if it does, the frame is too tall.
Of the ten or twelve bikes I've owned over the past couple of years, the ones that fit me the best for long rides had tall frames...standing flat-footed over the frame, up against the back of the stem, the top tube was lightly brushing my jeans...no daylight between me and the frame.
A taller frame makes it easier to get the bars up level with the saddle, ideal for a four or five hour ride. On a road bike, having the high part of the bars level with the saddle puts the "drops" level with the top tube, making it comfortable to ride on the drops for extended periods of time.
At the other end of the spectrum, for a 30 minute crit or time trial, a pro cyclist gets a tiny "mini" frame, enabling him to drop the bars three or four inches lower than the saddle, getting an extreme "aero" position...the best position for a time trial, and the WORST position for commuting in heavy urban traffic.