The mat should cover about 1/3 of the enclosure, so you can find the dinension yourself that works. I get the ultratherm heat pads off reptilebasics.com... they still need a thermostat, despite saying they don't need one in the description. They do get over 120 degress easily.

If it's plugged into a good thermostat, your snake won't get burned. Most reliable thermostat in the hobby is herpstats. The herpstat 1 basic works fine, and they're having a sale right now too.