Iron oxide nanoparticles (NPs) are promising materials for use in many applications, including new cancer treatments and in cleaning water, because they exhibit size-dependent magnetic and absorptive properties. NP properties are caused by structural attributes of the NPs, like surface disorder and cation vacancies. However, NP synthetic methods also impact structure, therefore properties, of NPs. Furthermore, the synthetic method is often changed in order to change the core diameter of NPs. Determining if properties are caused by the dimensions of the NP is impossible if there are also structural features present in the NP caused by the synthetic method, like grain boundaries or polycrystalline shells. In Chapter II of this dissertation, we show a new continuous growth synthesis of spinel iron oxide where the diameter of NPs is changed by the amount of precursor added to the reaction, meaning the only structural feature changing between the NPs is size. Continuous growth, therefore, can be used to probe the impact that size has on NP structure and properties. We report that saturation magnetization of NPs produced from continuous growth is size-dependent and higher in magnitude than NPs of the same core diameter made by other syntheses. In chapter III of this dissertation we determine nanoscale structure by Pair Distribution Function (PDF) analysis of Total X-ray Scattering data of NPs isolated from the reaction with core diameters between 3-10 nm. In Chapter IV of this dissertation we monitored the growth of NPs in situ with Total X-ray Scattering to gain insight on the structures of NPs while forming. In situ measurements of Total X-ray Scattering data gave insights into how precursor oxidation state influences the structures formed during formation of NPs, with more oxidized precursor giving a more oxidized product and a reduced precursor yielding a more reduced product even though the NPs formed by either method are indistinguishable by ex situ analysis. This dissertation includes previously published and unpublished co-authored material. / 2021-04-30