Flame-formed just-nucleated carbon nanoparticles, with sizes of about 2-10 nanometers, have been the object of increasing interest over the last decades not only because of environmental concerns but also as new procedure for synthesis of engineered nanoparticles. In this work, we present an experimental study on synthesis and characterization of carbon nanoparticles generated in a laminar premixed ethylene/air flame. The production of carbon nanoparticles of different sizes and properties is achieved by changing the particle residence time in the flame, i.e., collecting the carbon nanoparticles at different heights above the burner. Particle size distributions, Raman, UV-visible and electron paramagnetic resonance (EPR) spectroscopies have been used to characterize the sampled particles. The size of the particles increases as the residence time in the flame increases, the particle size distributions changing from a unimodal to bimodal. Chemical and structural modifications are retrieved by Raman and EPR analysis. Raman spectra show the G and D bands, typical of disordered carbonaceous materials. Their relative intensity and band position changes during the growing process and are used as index of structural changes. EPR spectroscopy, a powerful tool to probe electronic properties of carbon-based materials, reveals the presence and superposition of multiple paramagnetic species. Persistent carbon-centered aromatic radicals are detected for all the sampled particles and an abrupt change in the EPR signal is observed as the particle distribution changes from monomodal to bimodal. EPR indicate a three-dimensional structural organization when larger particles are formed. Optical proprieties are retrieved by UV-visible spectroscopy which, combined to Raman and EPR spectroscopy, seems to be powerful diagnostics to monitor particle clustering and to control the process of engineered carbonaceous nanoparticle production.