One of the tenets of graphic design is to leave a little room around your artwork. If you take things right to the edge of your poster, like taking a paragraph right to the bottom of the page, things just look crowded.
We like to see about 1-1/2 to 2 inches of space around a poster. You’ll see that we do this on our poster templates, and it’s just good graphic design. If it means that you make your poster a little bigger, that’s what you ought to do. It will look better, and more professionally present your research.
We can make you a poster of any size, by the way. If you need just a little more room on a 36×48″ poster, you can make it 50″ long for a few bucks more. You don’t have to go to 56 or 60″. Oh, and the trick to adding just a few inches to the size: select all, edit>cut, change the page size, then paste it all back. If you just change the page size, PowerPoint stretches everything.