I looked it up on the net and found the following: Measure from the base of the palm to the tip of the middle finger. When between sizes go the smaller as neoprene will stretch. Small = 6.75" Medium = 7.5" Large = 8" XLarge = 8.5"
You can go to the maker's website and print a sizing guide that helps make that decision. There are different guides for women and for men. I have large women's hands and got a 9. Sometimes I wear an 8 but I've heard these gloves run a bit small. I wear a large in rubber surgical gloves too.