Majority of ski goggles are manufactured in standard size (M) and tend to have a universal fit for both adults and teenagers. This size fits majority of people and it's very likely it's going to be the correct size for you as well.
Large size (L) goggles have wider, taller and deeper frame size that fits larger faces.
Small size (S) goggles, on the other hand, have narrower, shorter, and shallower frame size. They fit kids and youth, as well as adults with smaller faces.