You can make your rice bag whatever size you like, I made mine around 10" by 7". Cut your fabric so it is either twice as long or as wide as you want it and then fold it in half (right sides together) and sew it on two sides so that just one side is left open like a bag. Turn it right side out. Put in enough rice so that it is fairly filled but not stuffed, you want it to be able to mold around your body. On the open end flip the raw edges to the inside, pin it shut and sew it closed. On all seams it would be wise to sew them twice to give extra strength. You don't want to end up with rice all over you bed. Now you are all done!
To use just place in the microwave and heat for 1 1/2 to 3 minutes. Heating time will vary with microwaves and how hot you want it. If it is for a child, I would probably only heat it for a minute and 15 seconds.
These are a nice addition to any home and make nice gifts. You can chose a special fabric that the recipient would really like. My sister gave one to each of her children as part of their Christmas gifts one year. I believe they were all out of cute different flannel prints. I have added them to first aid kits that I have made for newly weds or just given them by themselves. It is a very economical, practical and easy to make gift.