There are times when the screen size for the destination of your presentation needs to be adjusted. How DO you change the slide size?
From the ribbon in PowerPoint 2013, simply select the Design tab. On the very right side of the ribbon, you will see a Slide Size button. Clicking on the two options that show when this button is selected will give you two standard settings. A third option is Custom Slide Size. Selecting this option allows you to make your slides any size you need.
In PowerPoint 2010, this option is in a different place, but not difficult to find. Again, on the Design tab, look to the very left. There you will see a Page Setup button in the Page Setup group. Activating this option opens a menu with the same options as mentioned above.