Most of large-size permanent room-temperature hard magnets contain rare-earth elements. Here, we present the preparation of a large-size rare-earth free hard magnet based on the loading of the ordered mesoporosity of a preformed ordered mesoporous silica monolith obtained by a sol-gel route with the Fe(NO3)3 salt, followed by a simple calcination in air. The investigation of the influence of the thermal treatment temperature as well as the Fe/Si ratio on the microstructure of the ϵ-Fe2O3/SiO2 nanocomposites reveals well-controlled ϵ-Fe2O3 particles size and size distribution. Their magnetic study allows us i) to specify the size of the particles with an enhanced coercivity and ii) to show a linear correlation between the magnetic coercivity of the materials and the mass percent of these particles in the nanocomposites. A ϵ-Fe2O3 based bulk hard magnet with the highest coercive field of 18 kOe at room temperature was obtained by this synthesis process.