There is a way to measure hands for glove sizing! Measure the circumference around the widest part of your palm, and the length from the heel of your hand to the tip of your middle finger.
The fabric between your fingers should slot in there against your skin, so you don't feel like a frog with little webs in between. You can either pull your sleeve over your glove and velcro at the wrist or vice versa, glove over sleeve to keep snow out (I personally prefer the second). P.S. Gloves are better for dexterity, mittens are warmer, generally.