The present study aims to obtain further understandings of vertical flame spreading phenomena by analysing the influences of soot and individual heat flux components on PMMA walls using large eddy simulation. Total heat flux consists of convective and radiative components, but it is not clear which one has a significant role in fire spread. The computational code used is an in-house version of FireFOAM 2.2.x, which has recently undergone specific development and validation for flame spread studies by the authors. The present study has conducted numerical simulations for flame spread and full wall fire configurations. By scale-up of the PMMA size from 0.4 to 1.0 m, the convective heat flux decreased by 41.4% at the location of the pyrolysis front, radiative heat flux increased by 86.9%, and radiative heat flux due to soot grew by 215.2%. As the pyrolysis height increases from 0.3 to 1.0 m, the convective heat flux decreased by 26.8% at the location of the pyrolysis front. The radiative heat flux increased by 96.8%, and its components of combustion of the gaseous fuel and soot grew by 55.9% and 233.3%, respectively. Moreover, the ratio of radiative heat flux to total heat flux increased by 66.5%, and that of soot to radiative heat flux grew by 73.9%. The contribution of soot to radiative heat flux almost linearly increased against the pyrolysis height and that was higher at a higher pyrolysis height.