I'm fairly sure it's 6 1/2" square. Although I checked and double checked, it's definitely not given in the directions. I came up with 6 1/2" by looking at the two squares next to it. They are both 3 1/2": 3 1/2" x 2" = 7", minus the 1/4" seam allowance on each (1/4" x 2 = 1/2") where they are sewn together would make that unit 6 1/2".