Goggle fit is based on approximate face size and also personal preference. In general, a small frame will fit someone with a smaller face. However, someone with a small face might prefer more coverage and therefore opt for a large fit. Goggle fit, for the most part, is not precise in the way that boot sizing is, for example.
The two most common types of goggle lenses are spherical and cylindrical (flat). Spherical lenses provide reduced susceptibility to fogging and typically better optic performance. Cylindrical lenses have the advantage of being lower profile and are typically lower in price, while still providing excellent optical clarity.
Some goggles come equipped to easily change out the lens. This is useful in changing light conditions. For example, if you start the day and it is cloudy, and then later on in the day it becomes sunny you will want a lighter lens in the morning and a darker one in the afternoon. Multiple types of lenses are available for goggles with interchangeable lenses. The lenses vary in tints and VLT - Variable Light Transmission - which indicates the percentage of light let through the lens. If a goggle does not have interchangeable lenses, it typically comes with a lens that is decent in all conditions.