With Windows XP, you can define custom paper sizes (such as 3×5 cards) by doing the following:
Start -> Printers and Faxes -> File -> Server Properties
In this dialog, you can define a 3×5 card. The problem is, with the LQ-590 and LQ-300 drivers, these paper sizes do not show up as options when you try and print. Any paper size below 4 inches in height will disappear from your printing options.
To get around this, install the LQ-570 driver (the one that is built into Windows XP). This driver will let you use a 3×5 sized form.