- Click the View tab, and then click Normal.
- Click the Design tab, select Slide Size in the Customize group, and then click Custom Slide Size.
- In the Slide Size dialog box, select Portrait.
- 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.
The video has been made by me and the text has been taken from microsoft blog