The recently emerged exceedingly small magnetic iron oxide nanoparticles (ES-MIONs) (<5 nm) are promising T-weighted contrast agents for magnetic resonance imaging (MRI) due to their good biocompatibility compared with Gd-chelates. However, the best particle size of ES-MIONs for T imaging is still unknown because the synthesis of ES-MIONs with precise size control to clarify the relationship between the r (or r/r) and the particle size remains a challenge. In this study, we synthesized ES-MIONs with seven different sizes below 5 nm and found that 3.6 nm is the best particle size for ES-MIONs to be utilized as T-weighted MR contrast agent. To enhance tumor targetability of theranostic nanoparticles and reduce the nonspecific uptake of nanoparticles by normal healthy cells, we constructed a drug delivery system based on the 3.6 nm ES-MIONs for T-weighted tumor imaging and chemotherapy. The laser scanning confocal microscopy (LSCM) and flow cytometry analysis results demonstrate that our strategy of precise targeting via exposure or hiding of the targeting ligand RGD on demand is feasible. The MR imaging and chemotherapy results on the cancer cells and tumor-bearing mice reinforce that our DOX@ES-MION3@RGD@mPEG3 nanoparticles are promising for high-resolution T-weighted MR imaging and precise chemotherapy of tumors.