Synthesis of monodisperse iron oxide nanoparticles (IONPs) with fine size-control over a wide range of sizes is of great current interest. In this study, the sub-nanometer scale size-control of monodisperse IONPs has been demonstrated in large-scale thermal decomposition synthesis by using the drying time of iron oleate as a single control parameter. The detailed investigation on the changes in the complex structures of iron oleate with drying time revealed that the relative population of Fe centers strongly bound by bridging and bidentate ligands increases, while the relative population of Fe centers weakly bound by crystal hydrate water and unidentate ligands decreases. This makes the number ratio of Fe species available for nucleation and particle growth continuously vary, making the drying time an excellent parameter for fine size-control. Controlling the drying time of iron oleate while other control parameters are fixed, the size of IONPs is finely tuned in sub-nanometer steps from ca. 6 to ca. 13 nm (synthesis at 320 degrees C) and from ca. 18 to ca. 25 nm (synthesis at 370 degrees C). The IONPs synthesized with this approach show a polydispersity of less than 5% and the particle size is highly reproducible. The effect of the drying temperature of iron oleate on the particle size, crystalline nature and superparamagnetic behavior of the synthesized IONPs is also investigated.