I have added a Print Widget in my Flex app, but I need some advice on how to print a map at scale in landscape mode to fit a letter size sheet of paper. For now the print widget is the default one that comes with the package. Any help is appreciated.
Configure your print widget to use a printing tool, it was introduced in ArcGIS Server 10.1.
You might want to create your own printing tool to use your own custom layouts. There's a tutorial for that.
Tutorial: Publishing additional services for printing—Documentation | ArcGIS for Server