I have the Denon 1600 and this is what the manual says when refering to DVD-A and DVD-V speaker settings:

Large: When speaker supports low-frequency (under 100 Hz) reproduction.
Small: When a speaker does not support low-frequency reproduction.
The size setting for the front speakers is automatically determined by the setting for the subwoofer.

The setting for the Subwoofer is only present or not present. If the mains are set to small and you remove the subwoofer the mains are forced to Large.