Hi dottie! Long time no "see".
I think somewhere around 6" by 12" would be good. That way it would wrap around a sore spot on the back, or hang over a knee easily without having to hold it there with your hand.
I use flax seed from the bulk section at the grocery instead of the rice. It doesn't stink and has a higher tolerance to the heat. I got this "idea" from a shop that sells these worldwide called the Lavendar Shop. I only have to heat it 30 seconds and it stays warm for 30 minutes. A couple drops of lavendar oil is very soothing too!
This is a great idea for a gift for guys too.
Take care,
Red