We present results from a numerical study of a coarse-grained model of diblock copolymer (BCP) thin films cast on a chemically patterned surface. The patterned surface contains chemical inhomogeneities with a repeat spacing length scale comparable to the linear size of the BCP molecules. We find that the orientation of the lamellae in the thin film and the overlap of the film morphology with the preassigned surface pattern is strongly influenced by the commensurability between the bulk unconstrained lamellar size lambda*, and the linear size of the surface inhomogeneities ru. PACS Numbers : 64.60.Cn, 61.41. +e, 64.60.My, 64.75. +g.