This seems to be kind of a difficult setting not many people use anymore, so thats probably why Microsoft hid it a bit

Others have had the same issues, and on this page is a short but complete description of how to set a custom papersize as a printer's default:
http://www.zan1011.com/paper.htmTried it with a freshly installed LX300+ II driver (Win7), and the new format showed up in the list of available Paper sizes, after I pressed the 'Advanced...' button of the 'Printing preferences...' of that printer.
I'd set the driver's 'Paper Source' to Tractor (and use the tractor-feed if possible), so tearing off the sheet doesn't move the beginning of the next page.
Don't have the hardware, so I can't try if it actually works, and you probably need to tweak the minimum margins to get all output on the paper, but that's just fine-tuning
