MDRN Size Guide
Fit: All boxer briefs fit snug and contour to the body. If you prefer a looser fit, then go up a size. If you have larger thighs, you may want to size up for a little extra room.
Measure: How to measure your waist - determine the point at which you prefer to wear your boxers, (some like to wear their boxers lower around the hips vs. a little higher on your waist) then run a tape measure around your torso, adding a finger in between to account for extra stretch/comfort.