Start with the size label.
Each product displays the size options supplied for that product. A label such as Size (Men) or Size (Women) identifies the source sizing category. Do not assume that a numerical size is the same across brands, regions or categories.
Compare before choosing.
Check the official size chart for the brand and model, then compare it with a pair you already own. Use the product style code to identify the correct model.
Default variants.
If a product has no selectable size, contact the store before ordering to confirm sizing and availability.