For now, I'm going to assume it's the 6180.
How to manually set the size of the speakers is shown on page 82 of the owner's manual. To do it automatically, see page 32.
If you are only running one pair of front speakers, then just use the "A" speaker setting and plug the speaker wires into the "Front A" speaker terminals on the back of the unit. If you have two sets of front speakers, then plug one set into the "Front A" terminals and the other set into the "Front B" terminals, and then choose the AB setting. You can use the "B" terminals to run speakers in another room. Check out the manual for more info on that feature.
Adam