There is no "small" or "large" speaker setting on the Onkyo 501. The only adjustments you can make are crossover, subwoofer mode, and number of speakers connected to the receiver.
I would set the crossover to 60Hz or 80Hz based on your spl meter and which one sounds best to you.
The Subwoofer mode tells the receiver from which channels to send low frequency signals to the sub. I have mine set to mode one, which is all channels plus LFE.