We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al2O3 thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6nm and a height of 0.7nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3–4-nm-wide crystalline Al2O3 strips where the deposited Co atoms are confined, the same features sustain. Only at 570K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5nm and height of 1.4nm) appears for growth at 450K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300K and annealed to 750K, in which the coalescence is eliminated and the two preferable geometries appear and coexist.