I LOVE half chaps. there are many different brands . I think you need to use a tape measure and go around the largest part of your calf, then measure the vertical part, I think to the ground. the size of your shoe does not amatter.
one thing I have learned. If you like to wear a spur, then try to get the half chaps that do NOT have any flap that comes down and covers the heel. it will interfere with the spur.
I like the Ariat , but prefer the Grand Prix. the zippers wear out, but I just get them mended at the local leather/shoe repair place. mine have lasted me many years .