In an AutoCAD environment when you need to have a custom sheet size, you can leverage the PC3 file to create what you need. Since Revit wants to use the Windows drivers, you need to be a bit more creative to add the custom sheet sizes you might need.
Go to the start button and select Devices and Printers.
Select the printer/plotter that you need custom sheet sizes for then check below the menu bar for a button that says "Printer server properties"
The dialog that comes up includes a check box to "Create a new form". This will be the new sheet size. Give it a width and a height. Make sure to give it an identifiable name (including the company name helps), then just click the "Save Form" button to add it to the list of available sizes when plotting.
If the printer/plotter is shared from a network location, make sure to complete these steps from the computer the plotter is installed on.