The magnetic properties of iron oxide nanoparticles govern their relaxivities and efficacy as contrast agents for MRI. These properties are in turn determined by their composition, size and morphology. Herein we present a systematic study of the effect of particle size and shape of magnetite nanocrystals synthesized by thermal decomposition of iron salts on both their magnetism and longitudinal and transverse relaxivities, r₁ and r₂, respectively. Faceted nanoparticles demonstrate superior magnetism and relaxivities to spherical nanoparticles of similar size. For faceted nanoparticles, but not for spherical ones, r₁ and r₂ further increase with increasing particle size up to a size of 18 nm. This observation is in accordance with increasing saturation magnetization for nanoparticles increasing in size up to 12 nm, above which a plateau is observed. The NMRD (Nuclear Magnetic Resonance Dispersion) profiles of MIONs (Magnetic Iron Oxide Nanoparticles) display an increase in longitudinal relaxivity with decreasing magnetic field strength with a plateau below 1 MHz. The transverse relaxivity shows no dependence on the magnetic field strength between 20 MHz and 500 MHz. These observations translate to phantom MR images: in T₁-weighted SWIFT (SWeep Imaging with Fourier Transform) images MIONs have a positive contrast with little dependence on the particle size, whereas in T₂-weighted gradient-echo images MIONs create a negative contrast which increases in magnitude with increasing particle size. Altogether, these results will enable the development of particulate MRI contrast agents with enhanced efficacy for biomedical and clinical applications.