This page is pretty discouraging: http://css-discuss.incutio.com/?page=PrintStylesheets. It says that the css landscape property is going away in CSS2, but might be back in CSS3 or something like that.
Anyhow, standards aside the follow works in some browsers. (IE7, Opera, and FF2 I believe, but I haven't tested it in a long time and I can't remember)
- @page { size: landscape }
Hope that helps. Let us know how much success you have with it, after you test in the browsers.