Open up your Zebra Setup Utilities
You need to install the same printer for a second time, click "install new printer"
Select the same printer that's already installed.
Click add new printer
After install, you should now see two printers. Now you need to configure the second printer to have the new "page size". Follow this article on how to do that..
ProTip - it's helpful if you put the page size in the printer's name, then you can easily tell which printer configuration is setup to print which label size.
You will need to restart your computer for the name change to take effect.