If you have beefy hands, width size is more important.
If you have long fingers, finger length is more important.
Step 1: Measure the length of your middle finger on the palmside.
Step 2: Lightly wrap a string/tailor's tape around the widest part of your knuckles.
Remember, your gloves will break in — BUT mostly in the width of the fingers and hand. They don't magically get longer.

Size | Middle Finger | Knuckle Size |
X-Small | 2.75" and under | 7.25" and under |
Small | 3.15" and under | 7.75" and under |
Medium | 3.25" and under | 8.25" and under |
Large | 3.3" and under | 8.5" and under |
X-Large | 3.5" and under | 9.0" and under |
XX-Large | 3.5" and OVER | 9.0" and OVER |