My most common use is to keep my head warm, and many times I've been glad to EDC a bandanna, although a thin SmartWool knit cap is small enough to serve that role, too.

It is often helpful to spread out a bandanna to keep together small items, such as emptying my pockets, etc.

As for size, about 22 inches square seems to be optimal for use as cap-style headgear. Smaller and you don't have enough cloth to tie a knot; larger and the cloth is bulky for tying a knot. So it is interesting to play around with it and see what you prefer. A larger bandanna will let you tie it so cloth hangs down the back of your neck to reduce sunburn.

Although cotton seems to be preferred, I'm looking for silk in the right weight/texture.

I try changing the way it is folded from time to time so it does not wear holes in it, something I learned the hard way.

Steve