In theory, "large" refers to a speaker capable of reproducing signals down to the neighborhood of 20Hz. As such speakers (not counting units with powered subs tucked inside) are extremely rare, there are cases where folks elect to use "large" if their speakers have a -3dB point of anywhere from 30Hz to 40Hz (the assumption being that there's not much in that range in most music and that the .1 track in movies is generally home to the major low frequency muscle).

Personally, my Studio/60's have a -3dB point of 30Hz and a -2dB point of 42Hz. I used them with no sub at all for over a year and a half, then added an SVS 25-31PCi and settled on a 40Hz crossover point as the best balance for music. Since I replaced the PCi with an LFM-1 I've been running them crossed over at 60Hz, and I've been quite pleased with the results.