Flexwatt Help

I want to install flexwatt in my 46x12x21" aquarium...
What size piece of flexwatt would be ideal for this aquarium?
Do I put the flexwatt under the aquarium or under the substrate?
Also, which thermostat is good quality but somewhat inexpensive?

i use 11 inch flexwatt and put it under the glass with a sensing probe on the flexwatt, a reostat is good and cheap. I would also get an infered thermometer to take acurate temps of the flexwatt.