I have a similar warmer to these http://hotsnapz.com/blog/sodium-acetate-heating-pads . I got mine from a different company, but it works the same way. All you do is click the metal disk to start the reaction which lasts for a while. Once it cools heat up some water to melt it again and its ready for use again. They can get pretty hot so you could put it under or near your reptiles and cover them with a blanket as others mentioned to better contain the heat. Mine is about 5 inches wide by 10 inches long. You can find bigger if you wanted.