Hello Robert Powell3,
Thanks for using Apple Support Communities! We understand you are looking for some help with changing the paper size when printing from your iPhone to your printer. We are happy to help provide some guidance on this.
We have a great article listed below that goes over the steps on how to user AirPrint from your iPhone. Take a look to see if these are the steps you are following when trying to print from your iPhone:
Use AirPrint to print from your iPhone, iPad, or iPod touch
If those are the steps you are following and still doesn't provide the option to change paper size, but your printer manufacturer offers an app on the App Store, you can download and install that app to see if it provides you the option.
We were also able to find some documentation from Cannon regarding how to change print size from your iOS device. Take a look at that, as well as how to contact them for further assistance here:
Set Paper Size With AirPrint on an iOS Device
CANON SUPPORT
We hope this helps.
Best Regards.