To the best of my knowledge there isn't anything that bolts to the factory tail rack.
What you can do is use the base from a Givi E181 which you can put pretty much any universal rack on. The E181 is designed to fit the factory tail cowl holes on classic Ks. You'd need to swap bases to go back and forth between the OEM rack and the E181 base but it's only four bolts to do the swap.
(http://www.twistedthrottle.com/ezimagecatalogue/catalogue/variations/4130-425x350.jpg)
(http://www.casporttouring.com/store/graphics/00000002/e180.jpg)
Size is a matter of opinion.
Here's what a 36L Givi looks like:
(http://home.comcast.net/~smithduck/tours/bmw5/bmw5album/slides/bmw5_2108.jpg)
And here's what a 23L Givi looks like:
(http://home.comcast.net/~smithduck/tours/111019_oh1/pics/DSC01687.jpg)