The present paper presents a tabulation of data on all 600 Galactic open clusters for which it is currently possible to calculate linear diameters. As expected, the youngest "clusters", with ages <15Myr, contain a significant (>=20%) admixture of associations. Among intermediate-age clusters, with ages in the range 15Myr to 1.5Gyr, the median cluster diameter is found to increase with age. Small, compact clusters are rare among objects with ages >1.5Gyr. Open clusters with ages >1Gyr appear to form what might be termed a "cluster thick disk", part of which consists of objects that were probably captured gravitationally by the main body of the Galaxy.