Each version of PowerPoint has different methods. The slides are created for compatibility as far back as PowerPoint 2007. Essentially, you want to change the page layout of the Master slide. Once that is done, apply that layout to all existing slides.
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 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.