How to Change the Default Print Size on a Mac

Creating a Custom Paper Size

1. 
   Click the File menu in any app that can print. You'll need to open the system Print window to set a custom paper size.
2. 
   Click Print.
3. 
   Click Show Details.
4. 
   Click the Paper Size menu.
5. 
   Click Manage Custom Sizes.
6. 
   Click the + button.
7. 
   Type a name for the custom size.
8. 
   Type the paper dimensions.
9. 
   Type the non-printable area dimensions.
10. 
    Click OK.
11. 
    Select your new size using the method above. Your new custom size will appear at the bottom of the Default Paper Size menu.