The openings on both ends are 5 inches squared, the inside measures about 7 inches. As for fuel use, you are correct in assuming use increases with addition of each additional burner. 2 burners use twice the gas as a single and a 3 burner would 33% more fuel as a 2 burner. It's not as bad as you might think. The forge is insulated with 2 inches of Durawool so heat loss through the shell is minimal. Restrict your forge openings and you can draw your burners down after you reach your desired temperature. But if you need lots of heat fast, the 3 burner delivers.....