WHY DO I NEED TO CHOOSE A SIZE? I THOUGHT ALL HARNESSES WERE ONE-SIZE-FIT-ALL!
These are all straps, and of course, adjustment is available—roughly plus or minus a size.
However, you have to keep in mind that some girls have a 55 cm waist, while others have a 110 cm waist. A single adjustment size simply won't fit everyone! Besides, it wouldn't look good, as slender girls would end up with a half-meter tail of strap dangling around their waist. And for legs, it's even worse!
That is why you need to choose a size. It doesn't have to be an exact match, just an approximate one. For example, if you wear size S, ordering an XS or M won't go wrong. Or, if you currently wear a size XL but plan to slim down to L soon, you can go ahead and order size L. It will fit both now and later when you lose weight. And even if you slim down to an M, the harness will still fit you.