This study investigates the impact of spatial resolution on QSM susceptibility mapping for brain iron quantification. We obtained 40 sub-millimeter resolution whole-brain QSM datasets, and simulated six levels of spatial resolution via k-space truncation. QSM-based iron quantification was performed at each spatial scale and compared against the reference. We found that estimation error was ≤ 5 ppb in the basal ganglia when the voxel dimension along all three axes was ≤ 2.0 mm. The finding suggests that scan time can be significantly shortened by reducing spatial resolution.