A fellow knitter asked me about this. I thought I would post it for anybody who's knitting mittens and aren't sure of the sizes needed. I don't like following a pattern. I'm one of those knitters who likes it broken down into inches. Knit by the inch. I knit charity mittens, so here are my measurements based on experience.
Use a 1-3 inch cuff for all sizes.
Distance from top of cuff to thumb opening:
Ages 3-5 1 1/2 inches
Ages 6-8 1 1/2 inches
Ages 8-10 1 3/4 inches
Small adult 2 1/4 inches
Medium adult 2 1/2 inches
Large adult 2 3/4 inches
Thumb length will be the same as the above.
Total length of mitten from top of cuff where stockinette begins to top of mitten:
Ages 3-5 4 1/2 inches
Ages 6-8 5 inches
Ages 8-10 5 1/2 inches
Adult small 7 inches
Adult medium 7 1/2 inches
Adult large 8 inches
Add one to three inches to these measurements including the cuff length you choose.
I stop a half inch short of these thumb opening to top of mitten measurements and divide these stitches in half for magic loop. You can place a marker at the halfway point if using straights or double points. K1, ssk, knit to three stitches before marker, k2tog, k1. Repeat for stitches after the marker. I do this on every row until I'm down to about 8 stitches, cut the yarn, draw through the loops, and fasten off. These decreases come out to about half an inch.
On any size chart you find online, they never tell you the distance from the top of the cuff to the thumb opening. This is going to be the same measurement as for the thumb length. Hope this helps.