Hello MaryNolte,
Thanks for using Apple Support Communities! We see you're having an issue with printing a full page with your Canon MX492, and we want to do all we can to help out.
To get started, have you updated the printer drivers for Big Sur compatibility? Update printer software on Mac
When you look at the print preview, are you seeing the full page, or just the upper portion which is what ends up being printed?
It sounds like you're using the printer as connected to your Mac. Have you tried AirPrint? This article has AirPrint info and shows your model is compatible: About AirPrint
We also found a support page from Canon, which may also help out: Canon Knowledge Base
Thanks for using the Apple Support Communities.
Cheers!