My feet are actually different sizes. My left foot being slightly smaller, because of club foot. I always go for size 12 though.
I went and got some running shoes the other day (£85!) and had my gait measured. Apparently my left foot (the bad one) is actually the good foot, with a strong and straighter position, whereas my right foot overpronates and has a slightly higher arc. And having club foot is the one thing which can bar me from service!