About the sizes: It's true that clothes range from M to XL depending of various factors; however, it is not something impossible to fix. For instance, it's possible to make an equivalence of "small of x1" = "large of x2".
Also, by asking the user to choose something he knows he's comfortable in, the website can get really interesting metrics to help determine the good size.
Finally, I guess it's always possible to give more detailed metrics of you (how much you weight, height, arms length, etc.). So, basically, it could be optional but if you're willing to spend a little time measuring it, it could be of a tremendous help. In fact, if I knew I could avoid big stores by taking a few measure of me, I'd do it right now.
Also, by asking the user to choose something he knows he's comfortable in, the website can get really interesting metrics to help determine the good size.
Finally, I guess it's always possible to give more detailed metrics of you (how much you weight, height, arms length, etc.). So, basically, it could be optional but if you're willing to spend a little time measuring it, it could be of a tremendous help. In fact, if I knew I could avoid big stores by taking a few measure of me, I'd do it right now.