I have to disagree. Unless your mains have better LF response than your sub (in which case: sell the sub), I cannot envision a time I'd want to set them for "large".
If you can find a response graph for your speakers, and preferably also for your sub, you'd set the crossover at the lowest freq. where the response is near optimal on the speakers.
The THX standard is 90hz, and it's generally a good place to start experimenting.