Given two different universes of different size, you could compute the entropy on the basis of the number of states of various types. I'm not sure if you should include information on their locations, or if that should be ignored.
Ignoring locations, this would make the universe with an extra 100 rocks have greater entropy and therefore be a priori less likely initially. You would also count universes as less likely which had more diverse conditions than ones which were uniform (all rocks for example).
Hal