Consider the Lenses
Once you’ve got your sizes figured out, when browsing the frames selection, make sure you know how the lens shape and size will impact the look and feel of your lenses!
Increasing either the horizontal or vertical dimension of the lens will also increase the thickness of the lens. Whether your prescription is + SPH (thickest at the center of the lens) or -SPH (thickest at lens edge), the cosmetic effect will be more drastic with a larger lens. With this in mind, lens shape and size is a big part of the overall style and character of your glasses--don’t be discouraged if you have your heart set on a pair of flattering frames with wider or taller lenses.
What can you do? Whether or not a new prescription has prompted your shopping for new frames, opt for a thinner, lightweight lens material. This will reduce any distortion of how your eyes appear behind the lenses, minimize curvature toward the edges of the lens, as well as increase the comfort of wearing the glasses.
On another note, keep in mind that the lens width (the first number of the frame size) will change quite a bit between lens shapes that differ drastically: for example, a round lens to a cateye.