The size setting for the speakers is actually not actually referring to its physical size, but the bass handling capabilities.

If you set your speaker to large, a full range of frequencies are sent to that speaker, if you were to set it as small, lower frequencies are not sent to that speaker and redirected to the speakers set to large. Because of this, you must have at least one speaker set to large. (Note, each speaker will have an internal cross over which will remove any frequencies which it is not capable of handling)

Here is a post which describes this in much more detail.

http://www.avforums.com/forums/av-amplifiers-receivers/1019893-crossover-speaker-settings-relation-bass-management.html