Size and older lenses
"Compared with your typical compact, it's enormous."
Bit of an apples-to-oranges comparison here surely?
"How is it with older lens series?"
The K-mount lenses work very well. Pentax-A lenses work normally in all modes, set the lens to auto and the aperture on the camera body. Focus and shoot.
Earlier manual aperture K-mounts use the lens aperture ring to set aperture and can be used in aperture priority or manual, with spot or center-weighted metering. The green button stops the lens down momentarily for metering and sets the appropriate shutter speed. Camera can make the lens stop down to what's set on the aperture ring to meter or shoot, but doesn't know what aperture was set.
Screw mount lenses with an adapter can't be controlled at all by the camera, so whatever aperture you select on the lens is what you get through the viewfinder and you use stop-down metering.
I believe you can use some of the medium-format lenses too with a suitable adapter, probably the same limits as M42 mount lenses.
All makes Nikon's famed backward compatibility look a little half-baked really. ;-)