Andy,
Go into the printer driver and under the Paper/Quality tab, click on the custom button.
Enter a name...I use one called "Variable" that I use for all my unusual paper sizes. Then enter the width and height, click on Save and the OK button. Your new paper size should appear in the driver window.
Each time I have a new paper size, I select my "Variable" paper, then Custom and then enter the new dimensions. This eliminated having too many defined paper sizes.
Hope this helps,
Jim