For frames and single vision lenses, ideally you would be able to find out:
- frame width (temple to temple)
- bridge size (nose width)
- A size (width of lens)
- B size (height of lens)
- temple length (arm that goes over ear)
- base curve / frame wrap angle
- panto (vertical incline of lens)
Frame manufacturer's web site has some info, at least [lens width - bridge - temple] are provided. To determine frame fit, you need sizes from current frame or model numbers from frames tested at a B&M store.
An optician will measure where your eyes are located horizontally (pupillary distance) and vertically in the frame (fitting height). This determines where the optical center (clearest vision) of the lens will be located in the frame. Most online sites for single vision lenses do not have an input field for fitting height, using averages instead. But they often have a "special instructions" field where you can enter custom instructions, including fitting height. HN thread on PD measurement: https://news.ycombinator.com/item?id=15154066
Progressive (varifocal) lenses are complex to buy even at B&M, never mind online, because fitting and use-case analysis matter more than with single vision lenses.
Since FramesDirect is owned by Essilor, they use Essilor lenses, which are well known. Other online sites don't say much about their lens manufacturer. http://eyeglasslensdirect.com (US) and http://specsdirect.ca (Canada) sell branded premium lenses at discount prices, online reviews are mixed. There are 1700 old msgs about cheap online glasses at https://groups.google.com/forum/m/?hl=en#!forum/glassyeyes
If the Luxxotica-Essilor merger goes through, retail stores like LensCrafters may start providing fitting/adjustment for online stores. A better outcome would be for independent opticians to unbundle their fitting/adjustment pricing, so you can buy your frames at one store (or reuse a high quality frame), a premium branded lens at another store/online, and obtain your frame fitting at a B&M optician.