I knit a lot of mittens for people last winter. I learned proper measurements by trying them on people. Most adults wear a medium mitten unless they have very small or very large hands.
Cuff Length: 1-3 inches, your choice
Distance from top of cuff to vee between thumb and fingers: 2 1/2 inches
Distance from vee to top of the middle finger (before my decreases): 4 1/2 inches
Thumb Length from vee: 2 1/2 inches.
Total length of mitten from cast on to finished top with decreases: 8 1/2, 9 1/2 or 10 1/2 inches depending on cuff height you choose.
Here's a handy mitten chart if you're making mittens for others for gifts. The charts don't say this, but the distance from the top of cuff to the opening for the thumb is going to be the same as the thumb length in these charts. If the thumb is 1 3/4 inches, the distance from the cuff to the thumb opening will be 1 3/4 inches, for example.