Each version of PowerPoint has different methods of doing this. If you are working with a very old version of PowerPoint, you may need additional assistance directly.
Essentially, you want to change the page size of the Master slide. Once that is done, PowerPoint should apply that change to all slides. Make sure you make your changes to BOTH the default slide, and the layout slide under Master Slide View.
For example, if your projector is static (unmovable) and is throwing the projection too low, you can create a page layout that is taller and move the text area to the top or bottom of the page, leaving the rest blank.
Please call or email if you need additional help. There is no charge for resizing a slideshow so long as the width remains the same. If you are changing the ratio of width to height, it will affect the slideshow dramatically and the show will need to be reformatted. There is a charge for this based on an hourly rate, but this process typically takes only an hour or two.