Stable aqueous suspensions of phospholipid-coated superparamagnetic Fe₃O₄ nanoparticles, or magnetoliposomes (MLs), were prepared and were separated by magnetic chromatography into size monodisperse fractions over a broad size range (50–130 nm). This development facilitated the first study of the size dependence of the MRI relaxation enhancements (relaxivity) of the ML suspensions. The iron oxide surface was stabilised by a primary layer of phosphatidylglycerol, but a range of lipids could be used to complete the bilayer, ensuring water dispersibility and control of ML surface properties. This work establishes for the first time that the outer lipid headgroup has a strong influence on the water relaxation time. Thus our approach provides multiple means to tailor both the biodistribution and the relaxivity of ML suspensions for biomedical applications as contrast agents for MRI, or as magnetically actuated delivery vehicles.