Size charts for shoes can be highly inconsistent, to say the least. This is why I measure every pair I sell by hand, multiple times.
That way you can compare them to a similar pair you own and decide if they will be right for you.
Length of the shoe
Design variations such as elongated toes, ultra high heels and overall style are taken into consideration. So a pair of winklepickers 11 inches long will very likely be the same size as a pair of flat loafers 9.9 inches long.
Width of the shoe at the ball of the foot
Knee length boots are the only style this doesn't work for so I measure the outside and deduct a margin. If you want more information about that when considering a purchase, drop me a line.