Here's the best way to choose a frame size on-line. Go out and buy a bike that sort of fits. You could go to a bike shop. Ride the bike. Outgrow it in terms of skill level and get another one. Repeat every other year or so for about 30 years. Now you have the experience to know what fits you and what doesn't.
Now you can order a frame on-line. But, because you've developed a relationship with the local bike shop (and paid for his modest boat

), hopefully you'll realize the benefit of personal service and avoid the on-line guys