Customizing paper size in Windows XP

Recently I upgraded a system from Win98 to XP Pro. But now I have a problem. The invoices system developed in Visual Foxpro6 which is installed only recognizes the standard paper sizes (letter, A4, etc.) I have discovered that in Windows 98 the invoices system used a customized paper size what is not available in the printer settings of Win XP. How can I set by default an customized paper size in Win XP Pro? is there another solution?