The "correct" leg position in dressage is that which allows the rider to effectively use her leg and seat, without drawing up the heel. A good saddle should place your leg correctly - not too far forward or back. Stirrup length plays a suprisingly small role and longer stirrups only really have a cosmetic benefit only IF your leg and seat are strong enough for a longer stirrup.
They say money doesn't buy happiness -- well happiness doesn't buy horses!
I came across this picture recently and thought it was a pretty good example of both horse and rider in a good dressage position:
Her back might be a little too rounded (or it could just be how to coat hangs), and maybe her hands could be a tiny bit higher, but other than that I think this is an excellent example. You can see that her ankle does fall slightly behind the girth, but not by very much, and squeezing with the calves will put pressure almost right above the girth.