Take your actual body measurements as they are more accurate than measuring over your clothes or shoes.
Feet: Use a ruler or tape measure to measure your foot (from the heel to the tip of your longest toe) Be sure to measure both feet. Many people have feet of different sizes. Fit to the larger of the two.
Remember, feet can expand over a quarter size during a full day's walk so ensure you get the right fit.
my walking boots were a bit too loose....thick socks didn't make enough difference and i was getting some rubbing.....these insoles are brilliant...they stabilized my feet in my boots and walking has become a pleasure again....expensive but worth it....