I believe the size I use is 16X28 or something similair. Size is going to be dependent on the Tortoises in question. The 16X28s are good for at least 2 medium sized Torts, possibly 3. The Kane heat mats that I have run at about 106 degrees without any controllers, which is perfect for me when the temps are at 30-40 degrees outside. The lowest temps my Redfoots experience are around 50 Degrees at night, anything lower and I turn the pads on. If you live in a cooler climate, you might have to use both a heat/farrowing pad and CHEs to provide both a warm spot and up your ambient Temps. Kane also makes a heat pad that has a rheostat on the cord, so you wouldn't have to mess with Thermostats being exposed to the elements. Hay is probably the best insulator I've found, although you could use aspen, or something similair. Make sure it is deep enough that they can just about cover themselves and you shouldn't have any problems. Just make sure everything is working properly and in good shape. Fire could be a real possibilty, and make sure you don't have any ant piles close by, they will migrate to wherever the heat is and cause problems