- Open Google Slides.
- Click the "+ New" button or directly open a blank slide.

- After opening a new presentation, locate the "File" option in the top left corner.
- From the drop-down menu, choose "Page setup."

- In the "Page setup" window, find the "Slides" section.
- Select "Custom" beside "Size" for slides with non-standard dimensions.
- Enter your preferred width and height for portrait slides. For instance, you can input 8.5 inches for width and 11 inches for height to achieve the US Letter portrait size.

- Once you've entered the dimensions, click "Apply."
- A confirmation message will appear, stating that the change is specific to this presentation. Confirm by clicking "OK."
