It's very simple : you just need to count how many eyelets are on both, left and right, side of your shoe. Shoe laces are sized by the total number of eyelets. If your shoes have 10 eyelets, you should choose 5 eyelets laces.
It is also important to consider the width of shoelace. On our website MyLaces.com, we offer 3 widths of shoelaces: thin city laces (for business shoes), medium laces (for bensimon's type of shoes) and wide laces (perfect for sport shoes).