Step 1:

Measure your bicep circumference. Don't flex. Nobody cares.

Step 2:

Measure your arm length. 

Size Bicep (Don't flex) Arm Length
X-Small 11" and under About 17.5"
Small 12" and under About 18"
Medium 12.5" and under About 18.5"
Large 13.5" and under About 18.5"
X-Large 14.5" and under About 19"
XX-Large 15.0" and under About 19"
XXX-Large 15.0" and OVER About 19.5"