A theoretical description of terminally anchored chain interphases is presented for the case of an interphase in contact with a multicomponent solvent composed of species of varying quality and size. This work is an extension of a model proposed by Lai and Halperin (Macromolecules 1992, 25, 6693) to calculate the global properties of a terminally anchored chain interphase, or polymer brush, in contact with a binary solvent in which each component is of monomeric size. Here it is shown that the solvent size, in addition to the solvent quality and chain surface density, has a profound influence on the equilibrium properties of the terminally anchored chain interphase. At low chain surface densities there are two limiting behaviors for a terminally anchored chain interphase in contact with a pure solvent depending on the solvent quality. Under good solvent conditions the interphase collapses to a constant, solvent-swollen thickness as the solvent becomes much larger than the monomer size of the terminally anchored chain. However, under athermal or bad solvent conditions the interphase essentially collapses upon itself in the limit of large solvent size. At large chain surface densities the interphase collapses with increasing solvent size regardless of solvent quality. The case of binary solvents is even more interesting. Under some conditions instead of a solvent-size-induced collapse of the interphase, an enhanced swelling of the interphase is observed with increasing solvent sizes over some ranges of solvent size, quality, and chain surface densities. The methods presented here are easily extended to ternary and higher order solvent solutions. The phenomena described here may be exploited for various technological applications of terminally anchored chain interphases, such as separations and control of interfacial properties. The approach outlined here yields a simple calculational procedure for qualitatively examining the various global properties of these interphases which are inherent to each potential application as a function of chain surface density and solvent quality and size.