Have you tried File/Print? You can check the "scale to fit paper" box there.
Of course, you'll need to specify that the paper is legal sized paper in the
printer properties -- just hit the "properties" button next to your selected
printer at the top of the File/Print dialog.
Assuming a normal PPT page size, that's what I'd expect it to do.
PowerPoint won't distort your slides at print time and filling a legal size
page with a normal sized slide would require distorting it.
On a copy of your presentation try:
File, Page Setup and set the slide size to 8.5 x 14
OK that, correct any size/position problems that causes (did I mention working
only on a COPY of your original) and then print that to legal size paper.