Small is the way to go, especially since you have a sub. Your sub is better equipped to produce the low frequency sounds, that is it's job. Taking the low frequency burden off of the towers should allow them to perform better, even if the towers are "technically" capable of producing the lower frequencies.
I suggest you look at your speaker's literature, or online, to find the low end frequency capability of the towers. The frequency response needs to be cited with a +/- X db value (generaly +/-3db) to actually be a useful measure. If the tower's low end is at or below 30 to 35 Hz +/-3 db you could set them to large (I sincerely doubt it, given the description).
Cheers,