If you check on the product page of each set of glasses and click on the "Additional Information" section, you can see the exact dimensions of each of our glasses. Many prescription frames and some sunglasses/reading glasses have a 3-number measurement on them (e.g. 57-20-135) which corresponds to Lens Width, Nose Pad Width, and Arm Length in mm. If you know the size of frames that tend to fit you, you can find the glasses that will fit you best on our site as well.
If you'd like some pointers on which style of glasses will be best for your face shape, you can find our handy guide here: https://truedark.com/how-to-choose-the-best-frames-for-your-face/