Laser based non-intrusive techniques are used to extract quantitative information about the combustion process taking place over vertically oriented thermally thick PMMA surfaces of different lengths under normal gravity and standard atmospheric conditions. Velocity and soot fields have been extracted. The PIV data show that the flow velocity depends on the length of the sample. In the case of the shortest length, the reactive flow is phenomenally laminar and the soot oxidation time is higher; this case shows the least soot production due to combined effect of higher residence time and lower fuel surface available. As the length is increased, the soot oxidation is limited due to increased pyrolysis from larger fuel surfaces, as well as due to increased values of buoyancy induced air velocity, as in these cases transition to turbulence is expected. The bubbling of PMMA during pyrolysis causes microjets of fuel vapors escaping the flame.