Would you buy second hand shoes online, not knowing what shoe size you were, particularly if there is the risk that they might have been stolen?
Probably not, when put like that!
With footwear you know your size. This is something established last time you bought some shoes, probably in a shoe shop where sales staff assisted you. They might have made you pace about and checked where your big toes were - that kind of stuff. They might have also tried you on the larger - or smaller - sizes to get it right. Although shoe shop sales staff are not rocket scientists, some of them are very good at what they do - find people the right shoes, ones that fit. They might take a dim view of those that insisted on only buying second hand shoes off the internet, without even bothering to know their shoe size.
Sure there is probably some chart out there on the internet that you can enter your inseam length into and get some shoe size out, but in taking that approach you may never know what a properly fitting is.
Bicycles are remarkably similar to shoes in that they have to fit YOU. Yes, you can adjust the seat and bars (much like you can adjust the laces on a shoe) but you still need to get the size right in the first place. If you already have a bike, e.g. a 19" frame MTB then you can get another one online of a similar size. But you are not there yet.