Just speaking in terms of "quantum size" (and not computing power, which is not understood yet), it depends on the graph of possible entanglements among the qubits. If it were a complete graph (allowing arbitrary entanglement) then the size of the relevant Hilbert space would exactly double with each additional qubit (i.e. one would need twice as many complex numbers to describe any particular wave function in the space). But the D-Wave chips operate with a fixed topology that (I think) is far from fully-connected (e.g. the 128-bit chip used a "Chimera graph", which they describe in blog posts and publications). The growth would only be truly exponential with sufficient connectivity (e.g. a planar graph would mean sub-exponential growth).