Change From Landscape to Portrait Mode
I love Google Slides for so many reasons beyond a presentation tool. Google Slides is great for creating multi-page documents. If students are creating posters, Google Slides is a great medium.
Use the File menu in Google Slides to choose "Page setup."
The default is "Widescreen 16:9." Click on this to change the slide size.
Change from widescreen to "Custom."
Swap Length and Width
The default size is 10 x 5.626 inches. Swap this to be 5.625 by 10 inches. This makes the slides portrait for posters.
Simply rotating the slides sideways produces a very skinny poster size. Try a dimension of 7.5 x 10 to create a portrait size that will print nicely onto paper.
Portrait Mode Ideas
Switching from landscape to portrait mode has unlimited possibilities. Here are a few ideas.
I love the open space of Google Slides to create things. Rather than creating tables and columns in a Google Doc, it is much easier to place pictures and textbox's for a newsletter into Google Slides. To distribute the Google Slides newsletter digitally consider using the File menu and choosing "Publish to the web" and distributing the published link.
Do a Google Image search for Infographics and you will see they come in all shapes and sizes. In my mind I tend to think of an infographic being in portrait mode. Google Slides is great for creating infographics. Don't feel constrained by the 10 by 5.625 inches. Set it to be whatever you want. Google Slides allows for multiple students to create an infographic on the same document, which not only is great for grading and peer review but also great for sharing student work if you have permission!
Tip: If you are creating an infographic in Google Slides and discover that you need more room try this trick; use Control A to select all. Control X to cut all the images and textboxes. Then go to the File menu and change the canvas size. Paste with Control V. If you resize your canvas with images on the canvas they will be stretched to match the new proportions and I'm pretty sure you don't want that.
If you want to create a Google Doc with a watermark, well you can not. The hack is to use Google Slides! You can add an image to the background of Google Slides. This is locked down. Then on the slides themselves, in portrait mode of course, add textbox's to create your text.