Epson Perfection V500 - Manual (Page 150)

Click the Color, Grayscale, or Black&White button to match the colors in the original.
Click the arrow to open the Size list and select the size of your original.
If you need to select a size that is not in the Size list, you can create a custom size. Click
the arrow in the Size list and select Customize. You see the Source Size window.