I have to dissagree with horseshorses08... I don't think keeping your thighs tight against a saddle is a good idea. Tensing your thighs translates to tensing your entire leg, to your body, to your hands...getting into the horse's mouth and in tern making the horse itself tense. I'm an advocate of just sitting deeply in your saddle and relax. Your horse feels every single bit of it.
Riding western us hardly different from riding english. Some minor differences but nothing extensive. Heels down. Shoulders back. Line from heel, hip, to shoulder. Soft hands while maintaining a straight line from the elbow to the bit.
Sit deep in your saddle, don't push your feet forward and brace off of your stirrups. Let your legs stretch down into the stirrup below and just think tall.
Stirrup length is easy to learn. While standing up in the saddle you should be able to have about a 2 inch space between your body and the seat.
Rule of thumb with saddle sizes is that, say if you ride in a 17inch english saddle...it's more than likely that you'll ride a size 15-16 inch western saddle. Best thing to do is try them out with an EXPERIENCED rider. Perhaps a trainer.