The mean size of flame-formed fluorescent carbonaceous nanoparticles was detected by in-situ time-resolved fluorescence anisotropy (TRFA) measurements. A long investigated coflowing laminar diffusion flame of ethylene was used as combustion system for both data comparison and for its importance in the field of particulate formation and emission control. From the analysis of the decay time of the fluorescence anisotropy, we estimated the diameter of particles selected by their ability to absorb and emit visible light, as a function of the flame radial position for several heights from the fuel nozzle. TRFA has allowed us to investigate intermediate classes of compounds between the UV absorbing particles and soot studied in previous works. These compounds have average sizes ranging from 10 to 20 nm. The technique proves to be applicable to the study of soot formation mechanisms, and specifically to follow the inception phenomenon based on the transition from the gas-phase to the solid particles.