1. Select the Design tab:

    Select the Design tab on the toolbar ribbon. The Slide Size menu button towards the right end has the slide orientation control.

  2. Near the right end, select Slide Size, and then click Custom Slide Size.

    Custom Slide Size menu option

  3. In the Slide Size dialog box, select Portrait.

    Click Portrait

  4. In the next dialog box, you can select Maximize to take full advantage of the space available, or you can select Ensure Fit to make sure that your content fits on the vertical page.

Source: Microsoft