Unfortunately your receiver does not give you many options. I would suggest, to start, setting all the speakers to small, then setting the subwoofer's crossover to it's highest frequency. Evaluate the performance. Is the bass location very noticable? The next option would be to set all the speakers to small and the subwoofer at 80hz. Again, evaluate the performance, take notes if neccessary. The third option would be to set the front speakers to large, run the speaker wires to the subwoofer (from the receiver) and from the subwoofer to the speakers. This only works if your subwoofer has "high level inputs". This would allow you to use the subwoofer's crossover. The rest of your speakers would be set to small in the receiver's menu.
Note: Crossovers in receivers and subwoofers are not brick wall filters. They roll off frequency response starting at the frequency indicated. For instance, if your receiver crosses your speakers over at 100hz, at 80 hz the sound would only be about -5db less than at 100hz. (Most receivers roll off frequency response at 12db per octave. 50hz to 100hz represents one octave as does 100hz-200hz, 200hz-400hz and so on) The same goes for your subwoofer. If it is crossed over at 80hz it will be about -3db at 100hz. In this instance you should be fine setting all speakers to small and the subwoofer at 80hz. The sub would make up for the dip in response from the speakers being crossed over at 100hz.
I hope this helps and is not too confusing.