We have to estimate how much heat you will need. If it is freezing outside, and you are using a thermostat, perhaps about 50 watts total is enough. One way to test that is to put a 50 watt light bulb inside the tube and power it with the tube all closed up and watch and see how hot it gets inside vs the temperature of the air outside. I'm guessing you want about a 25 degree C rise above ambient.
You need to understand ohm's law and the relationship to power. You can work backwards from the power you need, which I"m guessing is 50 watts. You multiply 50 watts x one half the value of one resistor (because they are hooked up in parallel) and then take the square root of the answer to get volts. You have to put this amount of volts (ACrms or DC) across the resistors to get that amount of heat. So tell us, what are the resistor values?