I have several of these. The Mountain Shop sells some that are 3 X 5 to 8 X 10. I get several hours of warmth from them. I place one on my stomach, not my gut but my stomach, warms the whole body.
Exothermic Crystallization Reusable Hand Warmers
The magic of these hand warmers is in the metal disc you see inside it. All you do is click the disc back and forth a few times and you'll see the clearish gel inside become opaque. As the opaqueness spreads, so does the heat! It's a crazy chemical reaction which we like to call AWESOME! Seriously, the gel is a supersaturated solution. Bending the disc starts an exothermic crystallization of the gel (i.e. it goes opaque and gets hot). And when it loses its warmth, all you have to do is wrap it in a towel and place it into boiling water for about 5 minutes and it resets itself (the crystals completely liquefy). Once it cools off from its boiling bath, it is ready to be used again. A great, fun way to keep your fingers warm this winter. They work great in pockets but just don't sit on them.