For that size room focus on sealing it up and using a tank. Burners are more for open rooms or rooms that are hard to seal. I'm working on sealing my grow room but before I can do that I gotta invest in rolling benches and that ain't cheap. I use a Titan Ares 4 burner. In a somewhat sealed room I go through 30 pounds of propane per month keeping levels at 1200 lights on.
Just so you know, a 4 burner co2 generator puts off around 10,000 BTU/hr of heat in your room. For me it's advantageous as I'm growing in a cool basement, except for when it's summertime in the midwest, then I gotta turn on air conditioning or the grow room gets 85+F.