To measure the size distribution of radioactive aerosols in the widest possible range, a complex system operating on different physical principles was designed. The system consists of a twelve-stage diffusion battery, a five-stage cascade impactor, and a set of three analytical aerosol filters with established permeability functions. The diffusion battery allows obtaining data on the distribution of aerosols in the range of 0.5–50 nm, the cascade impactor in the range of 0.8–20 μm, and the standard analytical filters in the range of 0.05–0.8 μm. This system was tested in a standard radon box. At a low aerosol concentration (less than 1000 cm−3), nearly 45–75% of radon decay product activity was associated with ultrafine particles with an AMTD of ~1 nm (the so-called unattached fraction). Additionally, a mode with a diameter of ~ 5–8 nm was registered. Larger aerosol particles were not caught by the cascade impactor, instead being deposited on the final filters. This shows that the particles are less than 800 nm in size (the first cut-off diameter of the impactor), which is in good agreement with the literature. When a high concentration of aerosol particles is created by an additional aerosol source, the unattached fraction is absent and maximum activity is observed in the cascade impactor and finishing filters.