You know...honestly this can be confusing. The de facto standard is to set them to small if you have a subwoofer. Period. What happens with some receivers though is the crossovers work differently. For instance, my friend's Denon 3803 did crossover differently than my H/K 630 did.
For him small or large much of the information still was sent to the subwoofer. So he could have large, but information was being sent as if it was set to small. Where as the H/K was different. If you didn't cross it over, the sub never got the signal. This could be due, in part, to the H/K's more robust crossover capability where it can break up small/large and crossover number settings for fronts/surrounds/center.
In either case though I'd recommend small. Unless your subwoofer is sub-par and your speakers would do a better job...the sub is meant to take those low signal Hz ranges...and the benefit is your amp has less strain on it, and there's more ampage for the rest of the spectrum to shine.
It's a win-win in my mind.
