You can add a separate stylesheet for when a user prints your page and design that to look best as a landscape print. Like this:
HTML Code:
<link rel="stylesheet" type="text/css" href="printstyles.css" media="print">
You can't force the page to print out in landscape though to my knowledge. It's the user's choice how they want a page printed.
I personally wouldn't want any website having any control over my printer settings.