Superparamagnetic and ferromagnetic gamma-Fe2O3 nanoparticles of different sizes (d(XRD) = 6-53 nm) were synthesized via a rapid, one-step flame spray pyrolysis (FSP) technique. Insightful information on the structural characteristics of gamma-Fe2O3 nanoparticles were obtained by combining extensively various characterisation techniques such as XRD, TEM, EDS, AFM, FTIR, and SQuID magnetometry. The morphology of gamma-Fe2O3 crystals transformed gradually from near-spherical shapes to 2-D hexagonal/octagonal platelet structures, as a function of increasing particle size. Along with the morphological transformtaion, the synthesized particles evolved from disordered cubic phase to fully ordered tetragonal gamma-Fe2O3. Saturation magnetisation (M-s) of the nanoparticles increased from 21-74 emu g(-1) as a function of particle size up to 13 nm, above which the Ms approached the values reported for bulk gamma-Fe2O3. The magnetic properties (Ms, exchange bias and coercivity) correlated strongly with the microstructure and the degree of cation vacancy ordering of the nanoparticles. The study provided an insightful view on the use of the flame aerosol technique to design magnetic nanoparticles with closely controlled physicochemical and magnetic properties.