It's important to measure your foot accurately to find the right size shoe. To work out the size and width you will require, please follow these steps:

Measure the length of your foot by standing on a flat surface with your full weight on your foot, then measure the distance from the back of your heel to the tip of your longest toe.

Add 10mm to your foot length to allow your foot room to move. This will give you the footbed length you need to find your size.

Measure the width of your foot across the broadest point. Compare your foot width measurement of the footbed width in your size to see if you will need a narrow or regular width. 

Example: Jackie's foot measures 240mm long and 95mm wide. Jackie adds 10mm to her foot's length to find she will fit comfortably into a Size 39 (250mm Footbed Length). At that size, the Regular Width footbed of 97mm will fit her foot properly.
If you are after a size chart you will find the link above the price of the product when on an individual product page.