The size can be made to conform to different parts of the body. I have made a number of different sizes. I prefer flax seed as it is is the most stable (from moisture, mold and critters)...I take a fine netting and make a bag slightly smaller that the size of the outer bag and once filled sew it shut...so nothing leaks out....the for the out bag, I use a very soft material flannel, terry cloth, etc...creating a pillow envelop, then drop the fill bag into to...if the out bag get dirty....toss, wash, dry, and reuse....I also ad lavender dried or lavender essential oils (few drops, especially for neck) the scent also adds to relaxation which helps in pain control.