Until relatively recently, the 'large' questions could only be answered qualitatively, since means of quantifying a fire size in engineering units did not exist. Eventually, it was recognized that since heat is the energy output of the fire, and scientific means exist for measuring energy, the problem may be soluble. The principles are clear.
Heat is measured in units of Joules.
What is usually more of interest is the rate at which heat is released, not the total amount. Prior to the 1970s, such ideas, while theoretically accessible, were not usable, since actual means of measuring HRR from fires were not available. The first instruments for HRR measurement started being available in the 1970s and they were bench-scale devices. Shortly after joining NIST in 1977, I was tasked to find a better way. Several years of exploration elapsed, and by 1982 I had invented the Cone Calorimeter , in its first iteration.