I like using all the space available to me on my posters. Big posters are easy to read and can get more attention.
At a recent conference, the organizers told the presenters that the poster boards were 8 feet by 4 feet, or 92 inches by 48 inches. But 92 inch by 48 inch posters didn’t fit on the board. The organizers had measured the poster boards from edge to edge, outside of the metal frame. The frame took up about an inch of space on all four sides.
When making your poster, make it 2 inches shorter in both dimensions than what the organizers tell you the space is. That will give you a little wiggle room if the organizers have miscalculated the available space. From my experience, this happens frequently.