The thickness dependence of magnetic properties has been studied in SmCo5 amorphous films with imprinted in-plane anisotropy for thicknesses ranging down to the nanometer scale (2.5-100 nm). The field induced in-plane magnetic anisotropy decreases considerably when the film thickness is below 20 nm. Analysis of the magnetic anisotropy energy shows that the decrease of the induced in-plane anisotropy is accompanied by the development of an out-of-plane interface anisotropy. Two different regimes for the coercivity (Hc) change are found: below 3.75 nm, the Hc decreases continuously with decrease of the film thickness, whereas at above 3.75 nm, the Hc decreases with increase of the film thickness. This change in Hc can be understood by considering the decrease of the short range chemical order for the thinnest films (<3.75 nm) and the relative decrease of the interface contribution with increasing film thickness. The changes in anisotropy have a profound influence on the domain structure, in which the angle of the zigzag domain boundaries decreases with the inverse thickness of the layers.