I have been making rice bags for over 15 years. We call them 'WARMIES'. Everyone has their own, all are different sizes. Daughter likes hers to be about 18-20 in' long, the size around is about the size of child's arm. I sew seams across the width to keep rice from shifting. My mother had one that she could slip her hands into.
Heat for 2 min. Heating to long will cause the smell to be stronger.