You really can't, but you can fake it. Change the slide background color to
"Black". Click the "Autoshapes" icon on the Drawing Toolbar, highlight
"Basic Shapes", and click the "Rounded Rectangle". Draw an object that fits
the entire slide (you might want to shrink down the zoom setting to easily
see the entire slide. You can even draw it slightly bigger than the actual
slide. Set the fill color to what you want, set the line color either to
"Black" or "No Line". If you want this effect on all slides, you can click
"View", "Master", "Slide Master" and do the above steps here. It appears as
though your slide size is not rectangle since the outside area is black.
Hope this works for you!