what size of sub panel do I need for a small in law unit
I would use a 30 space panel, because that is a cost sweet-spot between "Make sure you never run out of spaces" and "sanely priced".  Electrical equipment is so cheap that there's no real economy in scrimping to the point of inconvenience.  And believe me, you will want to upgrade things later, if not in the near term, in the far. 

The panel does not need a shutoff switch if it's in the same building. Generally if a shutoff switch were required, people simply get a panel with a main breaker; it's cheaper, but nevermind that.  

If the panel has a bus rating or main breaker, that does not matter (as long as it's equal or larger than the feed breaker). So don't be bashful about getting a 150A or 200A panel.    The important thing is to get plenty of spaces. 

I suggest a 60A cable feed and 60A breaker from the main panel.  The cable to use is #6 copper or #4 aluminum. (Aluminum starts to make sense at these large sizes).