Start Google Drive
Click New, Google Slides
or New, More, Drawing
Go to File, Page Setup. I used Drawings in the screen shots, but the process is exactly the same in Slides.
Choose Custom from the drop down under Standard 4:3
Change the width to 8.5, and the height to 11, and click OK
In Slides, go to Slide, Apply Layout, and choose Blank if you want a totally blank page to start
That's it! Now you have a portrait page to place pictures and text wherever you like!