As a general rule, a man’s measurement is the same as his size. Where small, medium, large, etc. are concerned, the best way to ensure proper fit is to know your measurements. A cloth tape measure and a full-length mirror. For best results take body measurements undressed or in lightweight clothing. Keep your arms at your side and have a friend take the measurements. When measuring, keep the tape measure snug, but not tight. Stand up straight!
You’ll need a friend to measure from your crotch to where you want your pants to end, breaking at the top of the shoe in the front and just above the heel in the back (you’ll need to be wearing shoes for this one). For easier method, find a pair of good fitting pants, and measure from crotch seam to bottom of pants.