Dimensions uses the default settings for the machine you are using to get the printer settings.
You can check the settings by following these steps:
Open the start menu
Type in "Control Panel"
Select Control Panel
Select Devices and Printers
Right click on the affected printer
Select Printing Preferences
In here you'll be able to select a new paper size to apply for when you use the printer
If the new size is needed to be used by all users, you can change the default settings for all users by following the below:
Open the start menu
Type in "Control Panel"
Select Control Panel
Select Devices and Printers
Right click on the affected printer
Select Printer Properties
Click on the Advanced tab
Click Printing Defaults
In here you'll be able to set the global default printer settings
You may not have access to the above options, if this is the case then please refer this article to your IT.