I guess it doesnt really matter because it will be attached to a thermostat like ALL heating sources should be.
I would get a mat 11x11 perhaps and put it on a timer (via the stat) so it comes on about 1200 midnight and off about 0500! To be honest i would invest in a temp gauge that reads temps at certain times and check just how cold your room gets. Chances are you wont need it. Good luck
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