Canon My Printer is just a troubleshooter which means it provides additional features to diagnose issues with your Canon printer, but as far as I know, you can use the Printing preferences found in OSX to customize the printing mode. Most likely, the printer is set up to print a size and you're using a wrong size for your files. When you use PRINT, there should be another menu to customize the printing process (size, layout, display mode, ink, colors, etc).