1. That the item of clothing in question is adjustable, often through straps, buckles, etc. However this level of adjustment doesn't always mean that it fits everyone. It just means it fits people within the range of adjustability.
2. That the item of clothing stretches. However there are limits to how far clothing can stretch. eg. If watching The Incredible Hulk on DVD, you know there is no way those pants of his stretch enough to keep covering his big green behind. He is simply too big.
3. That the item of clothing both stretches and has adjustable straps and buckles. This gives a much bigger range of comfort.
However "One Size Fits All" doesn't mean that the clothing in question is comfortable to wear - or fashionable. Often it means you will feel like a sausage being squeezed, as says one of the people in the video below which shows women of different sizes trying on various so called "one size fits all" pieces of clothing. As you can see in the video the clothing often doesn't really fit and is also uncomfortable.
I will say however that there are a few articles of clothing out there that do actually fit almost anyone and don't have to be adjusted or stretchy. They are:
Certain types of Wrap Dresses (not all wrap dresses are capable)
Jewelry (depends on the jewelry)
Basically just the clothing that by their very nature are adjustable to many different sizes.
I am not predicting there will be a huge increase in the number of people out there wearing capes or cloaks however - not unless we see a huge drop in global temperatures. (Old fashioned cloaks are super warm to wear in the winter.)