Let me try again.

 

Click the Start button, click on Settings, Click on Printers and Faxes.  From the menu bar, click on server properties.   A dialog box will appear and there is a check box for 'Create a new form' , clicking on this will allow you to enter a custom paper size.  Be sure to give this form a new name or you will overwrite the current form.  Then you might be able to use this new form as a custom paper size.  I did some research with the printers I have installed on my computer, some of them did not allow for custom printer size.