Congrats on a nice system. The crossover is the point that the receiver begins to roll off the low frequencies from your speakers and redirects them to your subwoofer. Since you dont have a sub yet( at least until tommorror), you should set all your speakers to large and not worry about your crossover. I am not sure how your receiver works, but if you dont have a subwoofer the crossover probably redirects low freqs from your small speakers to the large.
Once you get your subwoofer, I would probably set all the speakers to small and cross them over at 80hz.
Good luck.