I use the Thermophore standard size heating pad (14x27). This isn't a table warmer. It's a heating pad that you lay on top of your client. (On top of the sheet, so the sheet keeps it sanitary.) It's also an intense heat (which is what makes it feel so good) so I also put two handtowels between the heating pad and the client, to disperse the heat slightly. (This is the same technique PT's use in medical clinics with the moist hydrocollater heating pads).
In the winter, I use it throughout the massage session. It's large enough to lay across the feet both prone or supine and cover both feet entirely. It's also large enough to cover the entire back (even on tall men). For petite women, it covers the entire back & hips at the same time. (You can put it on the feet during back massage, on the back/hips during leg massage prone, on the abdomen during leg massage supine, and on the feet again, during arm & neck massage supine.) My clients love this heating pad!
I don't know if you've ever seen a Thermophore. They were originally designed for home use, and they have a handheld on/off switch. The idea was for a person to press the switch and keep it on until it felt too hot, then release the switch, allow it to cool a little, then press the switch to heat up again. This provided cyclical therapeutic heat applications. In the massage setting, you obviously don't want your client to have to mess around with pressing a switch. So, my colleagues & I override this function by putting a strap over the switch to hold it down (and keep the heating pad on) throughout a massage. Then, we use towels to disperse the heat, so it doesn't get too hot.
Why go to all this trouble, you might ask? The high heat ability of this heating pad makes it exceptional. It's a much higher heat than electric blankets, table warmers or other heating pads I've seen. That high heat simply feels better (and it relaxes muscles better - making our work easier).
If you choose to buy one, I recommend buying the original. (They now sell one with a digital thermostat that you can set for a certain level of heat. But like all products I've seen with thermostats attached, the thermostat's not accurate. It turns itself off long before it reaches a therapeutically high heat.)
Here's the website, if you're interested:
http://www.anyotc.com/index.asp?PageAction=VIEWPROD&ProdID=6617
P.S. Personally, I don't like the feeling of my feet heated up inside plastic. It makes my feet feel sweaty and sticky.