What nap roller should I use for primer?
Generally the smoother the finish the smaller the nap.  The nap on the primer wouldn't matter in this situation.  I doubt your walls were done with perfection - no offense (none of mine are either).  So you need a certain amount of primer to get to the point of painting.  Meaning if you use a smaller nap for primer you will just have to make more swipes to get to the same point.  Most painters I know won't prime with anything smaller than a 1/2 inch.