The best way to simulate the sizing guideline below, without having the actual bike in person, is to take the length of the inside of your leg (your inseam) and compare this to the Standover Height (distance from the ground to the top tube) on the bike geometry. As shown below, you should have minimum 1 inch of clearance between your crotch and the top tube.
Look for geometry information like this on the Sizing/Geometry tab. Notice, dimension E is the Standover Height referred to above. For conversion, 1 inch = 25.4 mm.
