I have been wearing glasses since I was 4 years old so I am not too worried about adjusting them to fit. I have become quite adept at adjusting my own glasses in the past 37 years. All the online stores I have looked at so far have the exact frame dimensions listed for each frame model. Like this one.
Notice the price difference on that page. The retail price listed is typical of what I have seen in the past week while glasses shopping at all my local retail stores. Almost a 500% markup!
That'* how they pay for the rent on the store at the mall.
Prescription eyeglasses require a prescription, yes. You need to have a licensed eye doctor decide what your eyeglasses or contact lens needs are. You take the slip of paper on which the doctor wrote your prescription, fax it or scan and email it to the online glasses store, and they make you a pair of glasses from the frames and lens options you select. Same way the store in the mall does it, except it is online instead of at the mall.
So far I have found some great prices that undercut the big chain stores (Pearle, Lenscrafters, Wal Mart, etc.) by selling at about one-fourth the price. I just haven't found any place online that accepts my insurance; and my insurance plan will absolutely not reimburse me for the purchase. If I didn't have insurance that will pay for a new pair of glasses each year, I wouldn't even consider going to a big chain store. Anyone that has to pay for their glasses out-of-pocket should at least check out what is available online.
Here is a good blog site I found with reviews of several online glasses stores:
Click on the store links to the left of the page.
I have found the identical frames a friend of mine bought and paid over $400 for at a big chain store, online they cost less than $100 including prescription lenses and a free hardshell case.