Epson Perfection V500 - Manual (Page 80)

If you need to create a scan size that is not in the Target Size list, you can create a custom
size. Click the arrow in the list and select Customize. You see the Target Size window.
Type a name for your custom size, enter the size, click the Save button, then click OK.
Your new size appears in the Target Size list.
In Professional Mode, you can fine-tune the size using additional tools. Click the +
(Windows) or
(Mac OS X) next to Target Size to display these tools.