You will get the best estimate if you look at the length of your insole (do not ever try to measure it, it will not be precise), just take a look at the size chart of any shoe brand you like and find the length in centimetres. You need to take a look at the US sizes in the size chart first and then find the length of the insole accordingly. For example, Vans shoes in a size US 7 have insoles with the length of 25 cm. Other brands may have a different US size attached to a particular length of the insole.
If you are still unsure about the sizing, we will gladly help you (phone number +44 2038071459, email address: firstname.lastname@example.org)