Google Slides and PowerPoint are both great tools when it comes to presentations. They both have some of the same features, although I think PowerPoint is a more robust program. In either case, they can be used for more than just a presentations. Both programs can be used to make newsletters, posters or flyers.
By default these programs are set up to contain slides - but did you know you can change the size of the slide to match any paper size?
In Google Slides -
Click File > Page Setup
By default the page size is Widescreen 16:9 - click on that option and then select Custom Size. Type in the size you want the page to be - 8.5 X 11, 11 X 17, 8.5 X 14 - whatever your heart desires. It can even be a size that doesn't fit your printer (you can save it as a graphic and send it to be printed at Office Max or other store that does poster size printing).
In PowePoint -
Click Design > Slide Size
Select Custom Slide Size > Click the down arrow on page size and select Custom. Type in the size you want and click OK.
Now that your page is set up like a normal piece of paper, you can add text boxes, graphics, shapes - whatever you want. It's easy to create a layout because everything is an object so it can be moved and re-sized easily. You can also use multiple pages (slides) to put together a large poster - just tape the pages together for the finished product.