if you go to the Performance or Nashbar website and navigate to the cycling shorts section you will find a button for sizing charts when looking at any pair of shorts. It gives the waist size range for the various sizes (s,m,l,xl,xxl) of all the manufacturers. You can tell from these charts which manufacturers sizes run small or large. I have found this helpful and I find I can wear a L in one brand and XL from others.

Good Luck.