The Default Paper Size setting in macOS is the size that appears first in the Page Setup and Print dialogues. It is recommended to check and change this if it does not match the size of paper usually loaded in the printer.
You may see "Job Stopped" messages if the Default Paper Size is set to US Letter and this setting is not changed when you print to A4 or another paper size. The Paper Size setting should always be checked prior to printing and changed if necessary.
You can set the Default Paper Size setting in Printers & Scanners, so that the preferred paper size appears by default in the Page Setup or Print dialogues.
Open Printers & Scanners.
Click the Apple icon, from the top-left of the Desktop.
Click on System Preferences.
In System Preferences, click on the Printers & Scanners icon.
In Printers & Scanners, the current Default paper size will be indicated at the bottom of the window.
The Default paper size may be US Letter which is not correct if printing to A4-sized paper. In this instance change the Default paper size to A4. If you normally use a different paper size, you can also select it here:
Click on the Default paper size menu.
macOS Sierra (10.12) example
Choose the paper size you require. This will be the size you normally have loaded in the printer.
The new Default Paper Size will be saved in Printers & Scanners.
Quit Printers & Scanners once the necessary changes have been made.