Measure the circumference of the part of your palm just below the knuckles, excluding your thumb (see image). Round up to the next whole inch, then add 1 to the measurement to determine your Keeper glove size.
ie. 7.5" -> 8" + 1 = 9. Thus, 9 would be the Keeper glove size.
Measure both hands, and order the bigger size if they are different. Remember: Goalkeeping gloves should be worn big, generally 1/2" to 1" over the end of your fingertips.