Hey corndog... I use a medium cruiser on my Ultra Classic... As stated above, you don't want it to hang over... I also found that you don't want it to go way up in front of you either... Maybe you can find a vendor that will let you try it on your seat before you buy?
Hey CornDog, I was just messin' wit ya on the measure, well kinda. I have the larger one, bought it for the front seat on my shovel, but honestly, I think on the back of a stock RK seat it might work, not sure about your Sundowner. I know teh measurements are on their website, so it should be easy to check out. I can tell ya, they do ride nice.