The size of residue molecules is crucial to catalyst design and petroleum use. An Athabasca tar sand vacuum residue was fractionated into 13 narrow fractions and an end-cut by supercritical fluid extraction and fraction (SFEF). Average molecule diameters and size distributions of the residue and its five SFEF cuts were determined from bulk-phase diffusion coefficients, which were measured at 308 K by a diaphragm cell. All five cuts show obvious polydispersity in size, with the end-cut possessing the broadest size distribution. A strong tendency of asphaltenes to aggregate suggests that the large size of the end-cut results from the aggregation of asphaltene molecules. The average hydrodynamic diameter of the end-cut was estimated to be 4.7 nm, as opposed to a range of 1.1-1.7 nm for the four narrow fractions. The average diameters of all five cuts can be correlated with their average molecular weight. In comparison to the size range of 1.1-4.7 nm for the narrow cuts, the feedstocks of the whole residue have a smaller size distribution of 1.4-3.9 nm. The attraction of heavier molecules at the beginning of the diffusion run and the disequilibrium at the end result in the smaller size distribution for the whole residue.