I made a pillow, and my top squares were an inch bigger. When I pinned the top square to the bottom, I just sort of formed a pleat with my fingers on the bigger square to make it match the bottom square and pinned, then sewed. I think if the square is smaller, it would be fussier to get the pleat just right and keep the edges aligned, if that makes sense.
Maybe try one of each and see what you think.