small. Dont let their stated freq. response fool you. I have all of my def tecs set to small. Leave the low stuff for your sub and not only will it sound better, it'll give your amps a rest.
I have 2 pair of BPVX/P's. Personally I have mine set to Large. I had them set to small for the first 6months, and just recently set them to large, and turned the gain on them up just a little. Made a world of difference. All depends what you are looking for in the way of bass. I find having them Large, adds a little to the rear soundstage. Not all speakers are created equal, so the all small rule doesn't always apply. Trust your ears and you'll be just fine.