Katie -- I'm not an expert, but Page Set-up has an option for 16:9.
But I'm wondering why? 16:9 is the size for a presentation, and PPT
will size your slides for display automatically.
It'll make the slide as large as possible on the display but won't
change the slide proportions. In other words, a default 4:3
proportioned slide will fill a wide-screen display top to bottom but
there'll be black panels at either side. A 16x9 slide will fill the
width of a 4:3 display but there'll be black panels top/bottom.
I'd guess that Katie's looking to fill the entire display.