This is what they call an Afterthought Thumb. You'll also heard it called a Peasant Thumb. You knit the mitten or glove to a certain point, then knit back and forth with waste yarn to hold your place for the thumb. I do mine this way. It's actually easier because you're not increasing or placing markers for a thumb gusset. They're knit like a tube that's closed on one end. The only downside to this is that it will give you a definite left and right hand glove. It's fine for adults and older kids, but may be a problem for kids who can't tell left from right hand. The advantage of mittens/ gloves with thumb gussetts or gores is that they can be worn on either hand. The proper measurement for this for an average adult is 2 1/2 inches from the top of the rib cuff to where you want to place the thumb opening. The thumb height will also be 2 1/2 inches. I spent a lot of time knitting mittens for charity and these are the right lengths for average size.
This is my favorite sizing chart for mittens/gloves. They never tell you this in these charts. But the distance from the top of the cuff (where you've stopped knitting the rib and have begun the stockinette stitch) is the same measurement as the thumb length in these charts. For example, the size 8 kids has a 1 3/4 inches thumb and the distance from the top of cuff to the thumb opening will also be 1 3/4 inches. I spent a lot of time using my adult kids and the neighbor kids as test subjects.