Particle size distribution functions (PSDF) and mean particle sizes have been determined in a laminar premixed ethylene/air flame with three different experimental approaches: photo-ionization mass spectrometry (PIMS), scanning mobility particle sizing (SMPS), and laser-induced incandescence (LII). The main goal of this investigation was the cross-validation of these three methods used at our institute for the determination of particle sizes in a great variety of flames or exhaust gases. We found good agreement between the three methods in the ranges where they are comparable as well as a complementary behavior for the different size ranges. PIMS and SMPS are able to measure the particle size distribution functions with good resolution. PIMS is favorable in detecting the smallest particles (6nm) and thereby able to detect even bimodal distributions of the soot precursor particles. SMPS and LII are suitable in the mid- and upper range of the particle sizes (2nm and3nm, respectively). LII offers the particular advantage of being a non-intrusive method. This makes it applicable in extreme environments, such as high pressure flames, as well as in very sensitive flames because no probe is needed.