These can be used as hot or cold pads. I generally make these in two sizes, one for the back and the other size for the neck. However, I can make them in whatever size you would like. Since the bags cannot be washed, each one includes a cover that is washable.
To heat the bag, simple stick it in the microwave for a short period of time. Once heated, the bag will hold it's temperature for 15 or 20 minutes. If the bag gets too dry (it has some natural moisture in the filling), simple put a cup of water in the microwave with the bag.
To use as a cold pad, simply put the bag in the freezer.