I'm 6'.2" and I ride a 23" frame and its perfect. You are only an inch shorter than me and the bike you are looking at is 3" shorter than mine. No real problem though as long as the seat is high enough and the handle bars does not cause problems for you. Actually a lot of the pros are riding frames smaller than what they would normally ride. It's up to you and how you feel. I'm somewhat of a mini "Clydesdale " (240 lbs = big bones) and I would not be comfortable physically or mentally if I was riding on a frame that is smaller than what I'm on now. Actually if your LBS does a proper bike fit, I dont think the 20" would be the frame for you.a 22.5 or 23" would be more your size. But what do I know, I'm still a rookie in this stuff...
Compact frames make it impossible to use traditional sizing methods. With a compact frame, you really need to "road test" the bike to verify its fit. First, stand over the top tube, just behind the stem. Even on a compact frame, the top tube ought to be within about an inch of your crotch, if your belt is up against the back of the stem.
When you are road testing the bike, your weight should be comfortably balanced between your rear and your hands. If all of your weight is on your rear, the top tube is too short. If you have too much weight on your hands, and feel stretched out, the top tube is too long.
Trek's website provides a "virtual" top tube measurement for its compact frames. This is an imaginary straight line from the seatpost to the stem, that allows you to compare a compact frame to a traditional frame. An "average" guy who is around 5'10", with a slack's inseam of 32 inches and a leg length of 34 inches will be comfortable on a bike with a "virtual" top tube of between 55 and 57.
A compact frame with a virtual top tube of 56ish provides a "fit" that is similar to a traditional bike that was listed as being a size 56 or 58 (the 23 inch range), yet because of their compact frames, they may be sold as a size 19 or size 21.
how far did your crotch clear the top tube (or middle of it if it's slanted)???
No slant ,I hate bikes with slanted top bars, I have just about an inch of clearance between the bar and my crouch. I also changed the adjustable stem to one that was about two inches longer so i could raise the handle bar some and extend it forward at the same time. The replacement stem idea came from my LBS during the bike fit and the bike fits me like a glove. I have no aches or pains. Prior to the modifications to the seat both vertical and horizontal along with the stock handle bar and stem setting, I would hurt in the arms, shoulder and wrist. Now I'm happy