Microsoft PowerPoint 2013 uses the default 16:9 widescreen size for all slides, but you can use the 4:3 standard size (which was the default in earlier versions of the software) or any other size you like. If you want to use custom dimensions, you can set your own width and height in inches or choose one of PowerPoint's custom presets for different types of paper, banners or overhead projector slides. When changing the slide size, you get the option to scale your slide's content to fit on the new slide size so the content doesn't get cropped.
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Select the "Design" tab and then click the "Slide Size" button in the Customize section.
Select the "Standard (4:3)" option from the drop-down menu to resize the slides to a 4:3 ratio. If your slides have the standard size but need to be resized to be widescreen, select the "Widescreen (16:9)" option.
Select the "Custom Slide Size" option to choose another preset or type your own width and height. Click the "Slides Sized For" drop-down list and choose a preset listed, or select "Custom" and type the desired dimensions in the "Width" and "Height" boxes. Select whether you want to use a portrait or landscape orientation.
Select whether you want to maximize your content's size or scale it so that it fits the new size without being lost or distorted. Click the "Maximize" or "Ensure Fit" button to make your selection and finalize the resizing.