If the sphere is expanding, then the result is the same -- a gradual decline in energy density per unit of volume. So the specific geometry of the universe is unrelated to Olbers' Paradox.
But there is strong evidence that the universe is geometrically flat at large scales, which in turn argues that it's infinite in size.
How does the apparent large-scale flatness of the universe make an argument for an infinite size? To explain, and just as a simplifying example, imagine that the universe is the surface of sphere. Now include the implications of the fact that we observe large-scale flatness.
Picture this -- imagine that the universe is the surface of a sphere, but the sphere's surface is perfectly flat. How large must the sphere's radius be for its surface to be perfectly flat? Think about how a sphere's surface is defined -- it's the unique surface that's equidistant from the center of the sphere, the surface that has a distance of R (R = radius).
To make both properties true -- to accommodate (a) that it is a sphere and (b) that its surface is perfectly flat, all you need to do is make the radius infinite.
In the case of the universe, with more dimensions, to achieve the measured large-scale flatness, all one need do is assume that the universe is infinite in size.