In general, I agree with the others for movies...you'll probably want to set them to small. I'd recommend that you experiment, though, and find what's best for you.
In your other thread, I believe that you mentioned that you're running a 2.1 system and have your SPL meter and set-up DVD all ready to go. In my system, I prefer having my mains set to small when I'm running 5.1 sound. However, when I'm just listening to stereo music, I prefer to have my receiver run my mains as "large" and also output bass to my subwoofer (I don't know if all receivers let you do that), and then I adjust the crossover on my sub. My bookshelf speakers are rated down to 53 Hz. I just happen to prefer the way my system sounds for music when my bookshelf speakers run full range, so I suggest that you check out the different settings available to you.