The rule of thumb I use for Under Tank Heater sizes is that it should cover about 1/3 of the floor but no more than 1/2. The others are right. You will need a thermostat or at least a dimmer to control the UTH. You can pick up a dimmer/rheostat for around $10-$15, and thermostats usually start at around $30. So it all depends on what you want to spend and what you are looking for in a thermostat...
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