Photoshop 25.1 wont print A3+ on Canon Pro 300

@dboy 

As it’s a MAC, you can reset printing system:
On your Mac, choose Apple menu System Preferences, then click Printers & Scanners
Press and hold the Control key as you click in the list at the left, then choose “Reset printing system” from the menu that appears.
After you reset the printing system, the list of printers in Printers & Scanners preferences is empty.
Reinstall the print driver, only from the manufacturer, not Apple.

Also take a look at the following article and check if that helps: 
https://helpx.adobe.com/photoshop/using/printing-color-management-photoshop1.html

You may like to re-install the printer drivers to check if that makes a difference?

Here is some general Adobe [Mac and Win] info on printing problems,
https://helpx.adobe.com/photoshop/kb/troubleshoot-printing-problems.html
Simple reset: hold the space bar when you open the print window. It resets the print dialogs

- - - - - - - -
If that doesn’t help -
Here is some troubleshooting advice:
https://helpx.adobe.com/photoshop/kb/basic-trouble-shooting-steps.html

I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

Hi @dboy, I see it's been a few days without any replies. Sorry about that! Did you get your issue sorted out? If you haven't tried this fix already, press and hold the space bar on the keyboard while selecting the option to Print from the File menu. This will reset the Print settings.

If that doesn't work, please give us some more details. In the Printer Setup screen, are you seeing options to set the A3+ Preset and as a Paper Size? Or are those options grayed out? Or do you set them but the A5 size is printed instead? Since it worked in previous versions, you might have to revert temporarily. Also, there is a new update to Photoshop 25.2. That might fix the issue, too. Let us know how it goes!