So, I quite understand that you are afraid of getting a pair of glasses that are not your type. And the solution may be easy, since nowadays, a lot of glasses are adjustable, and you could see some size options before closing the deal on the web page. If you are unsure, you could contact their salesperson for some info. Hope you get things right!
If the eyeglasses are inclined to fall down from the nose, the eyeglasses may be too wide which you need to do the adjustment. You could just use the screw to do the adjustment. You could also let the client in the real store of the eyeglasses to help you do this to make the eyeglasses be suitable for you to wear.