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.
I noticed that my Mac could print to the iX6820 via AirPrint (sans Cloud Print), so I extracted an old working PPD (circa 2016) from the Mac and started manipulating it until I got it working with ChromeOS (mostly having to remove some statements in the file that ChromeOS didn't like or understand).
To my surprise, this actually started to work, and I got some basic printouts directly from my ChromeOS device via AirPrint. It took a bit more editing to get most of the printer features and paper sizes to show up and work properly.
Having been through that experience, I think an updated procedure could be used today as outlined below. You'll need a Mac to do this...
1) In a terminal window on a Mac, run the "ippfind" tool to scan your network and display all of URIs for any IPP printers it finds. The output will hopefully include your iX6820 printer. On my system, I got a URI that looked like
$ ippfind ipp://canon-ix6820.local:631/ipp/print
2) On the Mac, locate the utility call "ipp2ppd". On my Catalina Macbook, this was found under
/System/Library/Printers/Libraries/ipp2ppd
3) On the Mac, locate the "AirPrint.ppd" file. On my system, this was found under
/System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/PrintCore.framework/Versions/A/Resources/AirPrint.ppd
4) Run the "ipp2ppd" program including the printer URI and the AirPrint.ppd file as parameters on the command line, and redirect the output (a generated PPD) to a file of your choosing. The command might look something like
/System/Library/Printers/Libraries/ipp2ppd ipp://canon-ix6820.local:631/ipp/print /System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/PrintCore.framework/Versions/A/Resources/AirPrint.ppd > Canon_PIXMA_iX6820_AirPrint.ppd
5) Edit the newly generated PPD file (like Canon_PIXMA_iX6820_AirPrint.ppd in my example) to remove lines that ChromeOS doesn't understand. At a minimum, you will need to comment out or delete the lines that reference color profile files (*.icc) that exist on the Mac but are unknown to ChromeOS. On my system, I had to remove the lines that looked like this
*cupsICCProfile Gray../Grayscale: "/System/Library/ColorSync/Profiles/Generic Gray Gamma 2.2 Profile.icc" *cupsICCProfile RGB../Color: "/System/Library/ColorSync/Profiles/sRGB Profile.icc"
You can optionally eliminate other lines that reference files on the Mac, but ChromeOS did not seem to mind one way or the other if I removed lines like
*APPrinterIconPath: "/Library/Printers/Icons/00000000-0000-1000-8000-84BA3B08E962.icns"
6) [OPTIONAL] You can manually edit some of the other settings in the PPD file if you want to tailor the appearance and behavior of the printer. For example, I modified the "ModelName", "Product", "ShortNickName", and "NickName" strings to say "Canon PIXMA iX6820" rather than the bland and generic "Canon iX6800 series". I also changed "DefaultPageSize", "DefaultPageRegion", "DefaultImageableArea", and "DefaultPaperDimension" to be "Letter.Fullbleed" instead of the "4x6" filled in by the ipp2ppd tool. Save all your changes and copy this PPD file to your ChromeOS device(s).
7) On your ChromeOS device, go to the printer settings (Settings->Advanced->Printing->Printers). If you haven't already added your iX6820 to the list of printers, ChromeOS should automatically detect the printer and have it show up under "Add printers to your profile". Pressing the "Set up" button next to the printer will prompt you for printer information, at which time you can give it your newly generated PPD file. If the printer was already added before and shows up under "Your saved printers", you can give it the new PPD pressing the three dots menu next to the printer and selecting "Edit". At the bottom of the Edit dialog box, you can give it your new PPD file and press "Save".
From this point on, you can print wirelessly to the iX6820 as if it was a locally attached printer. 
In regards to 13x19 paper, you should be able to add your own custom media sizes by editing the PPD file. You will need to add new lines defining "PageSize", "PageRegion", "PaperDimension", and "ImageableArea" for your desired media size, copying the format from the other parameters having the same name. Note that the measurement units are in points (72 points per inch).
Oddly, when trying to print from ChromeOS, the drop down list of paper sizes do not use the names given in the PPD file, but rather get renamed to things like "oe photo-I", "Index-4x6 (Postcard)", etc. Based on my limited testing, this appears to be a cosmetic quirk of ChromeOS, and the resulting prints seem to honor my PPD specified dimensions despite the odd names.
