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.