How can I change from letter to legal size documents?

Please refer to the Wiki Documentation for the complete Scanning reference.

This is done through the twain or ISIS settings for your scanner. To access these settings select “Display Scanner Settings” from the “Scan” file menu. Next click the “Run Job” button and before the scanning process starts your scanners TWAIN or ISIS settings dialog box will appear.

Every TWAIN or ISIS dialog is different, but any of them have a clear option for changing page size or auto detecting page size.