Everhat,
Setting your speaker to "Large" or "Small" really has nothing to do with that speaker.
Your receiver lets you set the Crossover for each speaker. This sets the lowest frequency that will be sent to that speaker. A cursury look at the Pioneer site doesn't show me the frequency response of your Center, so you might try setting its crossover at 80hz to start.
But if your receiver doesn't send anything below 80hz to your speakerr, what happens to those signals below 80hz? That is where Small/Large comes in.
If you have your speaker set to Large, those signals below 80hz are simply lost.
If you have your speaker set to Small, those signals below 80Hz are routed to your subwoofer.
This becomes even more important for your Front Left/Right, as their signal contains most of the bass. If you set them to Large, you'll likely miss most of the bass entirely. (Your Center speaker gets a lot of voice/dialog, so not as much bass as your Front L/R.) So unless you have a system w/ speakers that cost as much as a good used car, you likely want to set them all to Small, and get a good subwoofer.
There are other settings to play with, and your personal preference should be your guide, but all Small is probably a good place to start.
