Do you like to try on clothes? For some people, a trip to the mall isn't complete without spending a few hours in a department store, trying on dozens of different outfits. For others, shopping for clothes is a tedious exercise best kept to the shortest amount of time possible.
Of course, if you're going to buy clothes, you want them to be the correct size. Sometimes this means you have to try things on. At other times, you might be able to pick out your usual size and just eyeball the garment to make sure it looks like it'll fit.
“One size fits all" is a phrase used to describe pieces of clothing or accessories designed to fit all people. Over time, it has also been used to refer to anything meant to apply in all circumstances.
In reality, one size obviously cannot fit all people. People come in so many shapes and sizes that it would be impossible to design something that would truly fit all people. Most “one size fits all" items are actually designed to fit most average people.
Thus, it might be more accurate to term these items “one size fits most." For marketing purposes, though, “one size fits all" is much catchier. Of course, those people who are larger or smaller than average would argue against these designations.
Some people do not like the “one size fits all" approach. They feel it treats all people the same when they obviously aren't. They also sometimes use the phrase in a negative way to describe something that was designed with too little attention to detail.
Although “one size fits all" clothing may fit most people, it won't fit all people well. It may be too tight on larger people and too loose on smaller people. It's often used to describe clothing meant to be worn loosely, such as coats, ponchos or skirts.
Accessories, on the other hand, might indeed come in one size that fits all. Necklaces, wristwatches and ties, for example, don't usually come in a variety of sizes. They may come in many different styles, but they're all designed to fit anyone who might want to wear them.