Most manufactures will specify easy guidelines on how to choose the size for their sandals when buying online. Depending on the material, shape, and usability of the sandals, you may need to one up or down the size of your sandals.  A general rule of thumb when buying leather sandals is to stick to your original size, because they rarely stretch over time. In the case of elastic or flexible sandals, you can go down to size because they take a while to break in and after a week or so, they will stretch to the size of your feet.