The soot size distribution is often determined by using ex-situ granulometers, after sampling of the particles. But the quenching of aggregation process during the sampling is difficult to control and raises the question of representativeness of the results (Ouf et al. 2010 [1]). For this reason, optical measurements are more suitable. Thanks to the approximation of Rayleigh - Debye - Gans for Fractal Aggregates which is well suitable for such particles, (RDG-FA theory - Dobbins et al., 1991 [2]), measurement of static light scattering (SLS) can be interpreted in order to determine a size parameter called gyration radius. The inversion of experimental data by this theory to infer the gyration radius of monodisperse aggregates has been recently validated (Caumont et al. 2010 [3]).