How to Change the Size of a Slide on Keynote

Open a new or existing Keynote presentation.
Click on the blue 'Inspector' button in the toolbar. Click the 'Document' pane then click the 'Document' button.
Click the 'Slide size' pull-down menu at the bottom of the window. Select the desired slide size from the pre-sets or click 'Custom Slide Size' to enter the desired width and height in pixels. Click 'OK' to confirm the size.
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