Canon Pixma Pro-100 problem using iMac High Sierra 10.13.6

Because I was on High Sierra 10.13.6 I found that I needed to update my driver and software.

I assume by that, that you were able to download and install the appropriate printer driver from Canon that supports the version of macOS High Sierra running on your iMac ... correct?

If so, then it may be possible that Canon either removed some printer features of did not fully implement them with this particular driver.

Some thing you can try is to fully reset the printer system on your Mac to see if this can help.

• Reset the Mac's Printing System to Fix Printer Problems - Lifewire

Another option would be to enable the CUPS interface on your Mac to allow you more control over this printer:

• Enabling CUPS Printer Interface for Mac - vend

This may or may not help. I have a PRO-10 so it is a little different. Go to System Preferences/ Printers and Scanners. See if there are two listings for your PRO-100. One is AirPrint and this one for me will not allow 13"x19" or the type of Photo Paper as you describe. If I choose the other selection, in my case Canon PRO-10 series 2, this is the software I downloaded from Canon. It will allow me to choose 13"x19" (A3+) and also the type of Photo Paper to use.