Epson Perfection V500 - Manual (Page 115)

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.
To size your image by a certain percentage, adjust the Scale value.
To change the way your image is cropped, turn Trimming on or off. (See Epson
Scan Help for details.)
To unlock the proportions of your scan size, click the lock icon. Then adjust the
marquee or enter new width/height measurements.
Once you have selected your scan size, you can click and drag the marquee to the area in
the image you want to scan. For details, see
Adjusting the Scan Area (Marquee) . The
marquee proportions are retained as you adjust it.