Although you cannot use different sized slides in one PowerPoint file, for the actual presentation you can link several different files together to create a presentation that has different slide sizes. The process to do so is as follows:.
Click Slide Show, Set Up Show. If you are using a wide screen monitor and have the resolution set in the bottom of the window that opens to a wide screen resolution you will get the side bars. If you change it to a 4 X 3 resolution such as 1024 X 768 what happens is that the slide will expand to fill the monitor selected.
Create the two Powerpoints (with your desired slide dimensions). They need to be properly filled in to have linkable objects and selectable slides. Select an object in the main PowerPoint to act as the hyperlink. Go to 'Insert - Links - Action'. Select either the 'Mouse Click' or the 'Mouse Over' tab. Select 'Hyperlink to:' and in the drop down menu choose 'other PowerPoint Presentation'. Select the slide that you want to link to.
Any slide that isn't empty should appear in the 'Hyperlink to Slide' dialog box. Repeat the Process in the second Presentation to link back to your main presentation Reference to Office Support Page where this solution was first posted.