A series of thin and ultrathin (< 500 degrees Angstrom thick) polycrystalline yttrium iron garnet (YIG) films has been prepared on quartz substrates by the pulsed laser deposition technique to study the effects of dimensional changes on the magnetic properties. The film thickness was varied from 100 to 3800 Angstrom. Saturation magnetization decreased rapidly in ultrathin films with decreasing thickness. The lattice cell of all samples was found to be slightly distorted though the elementary cell volume was one of a bulk crystal. The Curie temperature of the films did not differ from the bulk YIG value. Ferromagnetic resonance experiments show evidence of a change from in-plane magnetization to a perpendicular magnetization in films thinner than 140 Angstrom. This reorientation can be explained by the magnetoelastic anisotropy contribution related to thermally induced strain. This contribution is likely to be the principal origin of the perpendicular anisotropy.