I think the ipp2ppd tool is asking the printer for its capabilities (via IPP protocol) and generates a PPD from there. I'm guessing the printer is simply reporting a limited list of the most popular paper sizes even though it's technically capable of...
TL;DR; The PPD file is the result of lots of experimentation and some hand editing. The gory details... When I found out that Cloud Print support was going away, I started experimenting with ways to make the ChromeOS built-in printer support work. ...
Yes you can still print to the Canon PIXMA iX6820 from a Chromebook without Google Cloud Print if you take advantage of the printer's built-in Airprint feature. You just need to make sure that Airprint support is enabled from the printer's web UI. R...