Concerning the outer sphere relaxation theory, the sensitivity of a T2 MRI contrast agent, expressed by the transverse relaxivity r2, depends on the diffusion length of water molecules relative to the particle size. For T2-weighted spin–echo imaging, theoretical concepts reveal three regimes regarding the r2 relaxivity depending on the nanocrystal size: the motional averaging regime (MAR), the static dephasing regime (SDR), and the echo-limiting regime (ELR). The r2 maximum corresponds to the SDR, which represents a small size regime. To verify the theoretical concepts and to adjust the SDR, tailor-made T2 contrast agents were synthesized by controlled self-assembly of superparamagnetic iron oxide nanocrystals (SPIOs) into raspberry-like nanoclusters with diameters of 30–200 nm using a PEG-based ligand. The results highlight an opportunity to optimize the relaxivity of T2 contrast agents by tuning the cluster size of SPIO nanocrystals.