It would not be possible to simulate a larger universe than our current one unless our understanding of physics changes. However in a simulation you could use multiple tricks to create a world which by all practical means IS larger.

For example video games use tools like occlusion culling or level of detail (LOD) to make stuff the universe appear high-quality but for a reduced processing cost. I have no doubt similar tools could be used in a simulated world to make the world appear larger then it actually is.