The size (volume) of a large tetrametallic molecular square, that has resisted characterization by mass spectrometry, has been determined by pulsed-field gradient NMR spectroscopy, a technique that reports on self-diffusion coefficients. These scale inversely with hydrodynamic radii, which in turn scale approximately as the cube of the assembly's mass. The technique has also been used to determine whether NMR spectral complexities observed for the new compound are due to contamination with chemically related assemblies, or instead reflect the intrinsic structural complexities of the compound itself.