Okay, the star diagonal is what you put the eyepiece into on the back of your telescope if you are using either a refractor or SCT. It's either that, or taking lots of Yoga courses to be able to bend your neck and back into positions most likely outlawed under the Geneva Convention.
Shaped like an 'L' for lack of better graphics on this forum, the light going through your scope is angled upwards to your eyepiece by either a mirror or prism. Prism diagonals are usually regarded as inferior to the finely made mirrors available today. Available in either 1.25" or 2", it is advisable to have one that is 2" if your scope has a 2" drawtube for the focuser. The AR-6 indeed does. Though you may do the majority of your viewing using 1.25" EP's, someday you will probably want to add at least one 2" EP to your collection for getting the infamous "Looking out the window of a starship" feeling! Most 2" diagonals available come with a 1.25" adapter to accomodate both sizes. The 'Cracker-Jack' toy that Meade includes is a 1.25" if memory serves me. I threw it in a black hole I keep in my home for things that annoy me.
Regards are Leviathans of 25"+ made for sale? Yes they sure are! There are ads and photos of such in Sky & Telescope, as well as Astronomy magazine. And the occasional photo from star parties of people balanced on ladders looking into the eyepieces of these monsters. These are usually made with trusses, rather than solid tubes, for ease(!) of assembly and transport - with the mirrors stored in a seperate box compartment often equipped with wheels.
Now I'll go back to my corner here and wait to be refuted on mirrors beating prisms, or something! Hahaha!!! Us amateur astronomers are always fighting over what works best for what. Kind of like people with expensive foriegn sports cars. Only me drive in outer space!