One authoriaitive person is David at Frank Harvey Hi-Fi, who has firm views on the subject, and with his huge experience of course I defer to him. However, I find that I prefer having "floorstanding" (though I still raise them on supports of various kinds) speakers for rear surrounds, and in the receiver menu I set the surround speakers to "Large". That is the case in three out of my four rooms; in the fourth and smallest room because of space limitations I have wall- mounted surround speakers, and they are small and set to "Small" in the receiver's menu.