Xsane scans just a corner of the document
 

I got my canoscan N1240U working on RH9 with the plustek backend. I edited the plustek.conf file and added the vendor ID and product ID en the Lamp Off time ( the lamp didn't go out )
But when I scan a document I only get a corner of the document .

In the scan windows of xsane there I see the size of the document and that says the document is 12.60 cm x 7.62 cm and thats is all i get when scanning . ( these measures are at the same place as where the status bar with %scanned will be when the scan button is pressed )

My question is : How do I modify the document size in the scan window ?