Open your Google Slides presentation
Click File
Click Page setup
Click the dropdown menu
Select Custom
Enter a vertical size, such as 8.5 x 11 or 9 x 16
Make sure the width is smaller than the height
Click Apply
Open your Google Slides presentation
Click File
Click Page setup
Click the dropdown menu
Select Custom
Enter a vertical size, such as 8.5 x 11 or 9 x 16
Make sure the width is smaller than the height
Click Apply