To Big of a box to Small of a Box

What happens to a speaker if you make the box to small?  What happens to the speaker if you make the box to big.  Vented and Sealed.  I have a customer that thinks I need to take up his whole trunk to make his speakers sound good.  They are 3 12s and all they need is 6cu.ft of air space.  And then vent will bring them to 39.7Hz.  I just want to be able to tell the guy the reason that we did not use his whole trunk.  I know that if you make a vented box to big that the speakers will move but, make like no sound.  I tried it with the box I built for 2 12 MAs and I put my kickers in there and the kickers sounded like crap I put the the MAs in there and they are still slamming I just don't want to make up some bs I want to be sure I am giving out the right reasons so I don't look like an ass.

Thanks