Stirrup length definitely affects your leg position but there are other things you can do to help you equitation. Here are some tips how to check to see if the length is good for you: How to Determine Correct Stirrup Length | eHow.com
It is helpful to shorten them a bit with jumping, but don't go overboard. Whatever is comfortable and provides a secure seat. I personally only go up one or two holes when I'm jumping 3' or higher, but that's because I feel most secure and correct in my position at those lengths, it's different for everyone.
I'm only 5'3", but my legs are long in comparision to the rest of my body. My show horse is a big boned pant, and even though he's only 15.3h he takes up good amount of my leg with his wide barrel. What really helps me with my leg postition is riding without stirrups. Before show season starts I do most my riding without stirrupts to get my form in tip top shape. There are also some stretches that can help you reach down into your heels. When you're on your horse stand up in your stirrups and strech down into your heel, you'll start to feel it in all the muscles running down your leg, it also helps with blance when done without supporting yourself with your hands. Another exercise I do you don't even need a horse, just a stair.
It's explained well here: Exercises for Horseback Riders - Associated Content from Yahoo! - associatedcontent.com
I don't think being taller is 'better' than being short when it comes to riding. I've been the reigning equitation champ in my region for 3 years now, it just takes time and practice to perfect each little detail.