I've dug up several studies. They all agree that the 30-40 liter cavity is ideal. But one also varied the entrance hole. Much to my suprise, if you open it up too much, it becomes highly undesirable. A 40 liter cavity with 15 cm ^2 entrance is the near ideal hive. But open that up to a 60 cm^2 and bees reject it and take a sub par 15 liter hive with 15 cm^2 hole.