I use my thighs down to my knees (that's the part of the leg in contact with the saddle) to balance my upper body. That lets me use the lower parts of my legs to ride my horse and kick. Guys who stand up use their knees down for gripping their horse, standing and supporting their upper body, so it's hard for them to kick. They tend to lean forward to ask their horse to keep moving, but when you do that and your horse gets short it tends to throw you down in your upper body when you throw. (David Key is heading here.)
Stirrup length is important, and the size of your seat affects that length. The longer the seat of the saddle, the shorter your stirrups should be. Your stirrups should be adjusted to bend your leg and bring it up to the swell of your saddle. If you have a saddle that fits you better and tighter you can get away with a longer stirrup length, because you're already up close to your swells.
I ride a 14.5-inch seat. When I stand up, I like to just barely clear the seat by about an inch or so. I ride with my knees bent and probably 40-50 percent of my weight in the seat of my saddle, but still keeping contact with the seat and swells of the saddle for my balance support and stability.
