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.