Our family loves heating pads in the winter time. A few years ago my SIL and her family sent each of us one for Christmas. We all fight over the microwave at night. We love to use them for ear aches, headaches, muscle aches, and my children love them especially for warmth! It's cold here in New England.
We love to make them and give them away as teacher gifts, or gifts for extra special people.
There is no right or wrong size. I don't use a pattern. I just cut rectangles and start sewing and filling.
We add instructions inside the gift bag. Heat for 2 minutes on high in the microwave. I put together little gift bags with a note on the outside saying, "to warm you up on cold winter nights." I can't wait to give these great, personal gifts away.