Ultra-small-angle X-ray scattering (USAXS) of agglomerated and nonagglomerated flame-made silica nanoparticles is investigated systematically for ubiquitous characterization of particle size and degree of agglomeration. Primary particle diameters determined from the USAXS particle volume to surface ratio were compared to those obtained from nitrogen adsorption (Brunauer-Emmett-Teller (BET)) measurements. Independent of the silica precursor state (vapor or liquid), production rate (5-1100 g/h), degree of agglomeration, or flame conditions (premixed, diffusion, or spray flame), there is excellent agreement between BET and USAXS for the average primary particle diameter. Furthermore, the USAXS data reveal the effect of the fuel and precursor flow rate for various vapor- and liquid-fed flame reactors on product primary particle diameter and agglomerate size.