Can not be done.
However, why do you need to?
If you are using PowerPoint to project...
The slide can only be shown as large as the projected image. Unless you
plan to rotate the projector during the show, your max height will still
only be the height of the project/screen. You are better off staying in the
projected slide size and ratio; just fill the height and don't worry about
the width.
If you are printing ...
Then just rotate the text and images 90 degrees and turn the printed page to
read it.
Bill Dilworth