You can set the left and right front speakers as large or small but all that means is ("large" setting) send deep bass to the speakers, or ("small" setting) send the deep bass to the subwoofer. It has nothing to do with the actual size of the speakers. I have a very capable sub in each room so I set my towers to small.
With a 5.1 system you usually want your surround speakers to the side because that's where those channels are expected in a video. With a 7.1 channel system you have both side surrounds and rear surrounds.
Check out some of the articles
here.