From Giro:

Ski and snowboard goggles come in various frame sizes to accommodate different face shapes and sizes. While specific sizing can vary between brands and models, goggles are generally categorized into three main sizes:

• Small: Designed for smaller faces, including children and individuals with petite facial features. Small goggles offer a snug fit to ensure proper comfort and for smaller wearers.
• Medium: This is the most common size and fits. Medium-sized goggles provide a balance between coverage and comfort. They are suitable for a wide range of users with average facial dimensions. Medium to large face shapes should shop for larger frames such as the standard Contour.
• Large: Geared towards individuals with larger faces, these goggles offer extended coverage for better peripheral vision and a comfortable fit. Large or extra-large sizes are ideal for those who find standard goggles too constricting.
• RS (Reduced Size): Giro offers goggles sized specifically for women and children. Giro's Contour RS is a goggle designed specifically for smaller faces, there’s a version for women and men. There are also youth-specific goggles such as the Buster and Chico 2.0.

How to Fit Your Goggles

A proper fit is crucial when it comes to goggles. Make sure your goggles form a tight seal on your face and use the adjustable strap to adjust the fit around your head or helmet. An improper fit can result in the goggles shifting around on your face and/or be prone to fogging and simply wiping the lens down with your sleeve can result in permanent lens damage (see Care section on page).