This work describes the use of mesoporous SBA-15 silicas as hard templates, for the size-controlled synthesis of oxides nanoparticles, the pores acting as nanoscale reactors. This fundamental work is mainly aimed at understanding unresolved issues concerning the occurrence and size dependence of phase transitions in oxide nanocrystals. Aqueous solutions of Fe(NO3)3.9H2O are deposited inside the pores of SBA-15 silicas with mesopore diameters of 4.3, 6.6 and 9.5 nm. By calcination, the nitrate salt transforms into FeOx oxides. The XRD peaks of nanocrystals are broad and overlapping, resulting in ambiguities of attribution to a given allotropic variety of Fe2O3 (hematite or maghemite) or Fe3O4. The association of XRD, SAED and Raman information is necessary to solve these ambiguities. The metastable gamma-Fe2O3 variety is selectively formed at low Fe/Si atomic ratio (circa 0.20) and when a low calcination temperature is used (773 or 873 K followed by quenching to room temperature once the targeted temperature is reached). The small size dispersion of the patterned nanoparticles, suggested on a local scale by TEM, is confirmed statistically by magnetic measurements. The nanoparticles have a superparamagnetic behaviour around room temperature. Their magnetic moments (from 220 to 370 mB), their sizes (from 4.0 to 4.8 nm), their blocking temperatures (from 36 to 58 K) increase with the silica template mesopore diameter. Their magnetic properties are compared to those of standard gamma-Fe2O3 nanoparticles of similar size, obtained by co-precipitation in water and stabilized by a citrate coating.