most fires do not burn steadily. the heat release rate ramps up, peaks,  and then drops down. a well established experimental correlation in fire science shows that the mean flame height ($L$) divided by the effective diameter ($D$) of the fuel source  is proportional to the heat release rate ($Q$) to the 0.4 power. The correlation does well for large fires but doesn't do as well for smaller fires or for mass fires (when $L/D < 1$). For typical fuels, under atmospheric conditions: 
$L = -1.02 D + 0.235 Q ^{0.4}$ ($D$, $L$ in $m$; with $Q$ in $kW > 0.8 kW$). So, for a $1 kW$ round pile of wood $10 cm wide, L ~ 14 cm$. Alternatively, $Q = [(L+1.02D)/0.235]^{2.5}$, so if $L$ and $D$ are known, $Q$ can be estimated.