I've bought from JingleBobs a few times. As it happens, I am about to do so again. As a company they are kinda dull - I order stuff from their website and it turns up in the UK a few days later. Nothing more to report. They send bandannas in a package small enough to fit through a letterbox, so I don't need to be home to take delivery, which is good.

Their product is good quality. I found their red silk to be quite dark and prefer their orange if you want it for signalling.

The 35" square are a good size. The diagonal is about 57". That is long enough to go around any part of my body, so it can serve as a belt or to bind wounds in first aid etc. It is big enough to make a sling for an arm.

The 20" ones for me are borderline too small. The diagonal is about 30", which is big enough to go around my head, but if I try wear it in traditional bandanna style the pointed bit doesn't really reach far enough to catch under the diagonal ties. I wish it was an inch or two bigger. However, it's fine for a head-band and many other uses.

They also do 42" square ones, which I found too big to be practical.

I prefer silk to cotton because it is lighter and thinner. A 35" silk is roughly the same weight and volume as a 21" cotton. It will adsorb about the same volume of water. However, having larger surface area it has more uses, and it dries quicker. Cotton is thicker, which is sometimes useful but you can double-up the silk. I don't see any point in carrying 21" cotton if silk is available.

20" silk is worth carrying mainly because it is thin and light enough to go in places that 21" cotton and 35" silk won't. Specifically, it will go in shirt breast pocket without being noticed. It's a great way to increase your every-day carry resources.

Uses include the following:

• A covering for the head to protect from sun-burn. Also as a sweat-band. Also to cover the back of the neck. Wet it to to cool by evaporation.
• In first aid, to clean up blood. Also as a bandage to secure dressings, or to apply pressure to wounds. Also as an arm-sling; to improvise splints; as a tourniquet (if you know what you are doing).
• As cordage: eg as a belt, to tie a car boot down, as a dog lead, or just about anything where 57" of parachord would be useful. If you cut it into 1" strips you get over 1000 inches of cord, that could be useful eg for building shelters.
• As a handkerchief to blow your nose on. Also as toilet paper (if you cut it up, 35x35" is roughly 49 5x5" squares).
• As a bag or sack. You can make a wide-mouth bag just by tying knots in the corners, which can be useful for preventing screws from getting lost; collecting nuts or berries etc. You can make a narrow-mouth bag by tying the corners together - it's worth looking up how to do that properly. You can use the bag to carry other supplies, collected food etc. Also as a crude glove, for warmth or protection.
• As a shield to protect from dust or dirt, eg to sit on. Also tie around the mouth as a crude dust-mask (may work better when wet). As a duster or "oily rag" to clean machinery, a car windscreen etc.
• As a towel for drying things. Also as a sponge to help get drinkable water from slow seeps, fine spray etc. As a funnel to help collect rain-water. (I suspect you could use it to collect water via condensation.)
• Use it to filter gross organic matter out of wild water as the first step in purifying.
• As a flag for signalling. Cut into small pieces, use for marking your trail. Cut into strips to form arrows and signs. Make it easy for rescuers to find your spot even if you are asleep when they pass.
• As clothing. You can form a loin cloth or bra, or a nappy. 35" is a bit small for a skirt, unless you are very slender, but it works as an apron.
• As a blindfold or gag. To bind wrists or ankles - it's prettier and less painful than cord. (It's surprising the sites you can find when you use google.)

I've probably forgotten whole categories but that should give an idea of their versatility. Some of the uses are destructive, so it can be beneficial to have two or more. Safety pins are often useful when working with cloth.