Bucket Hat

Hello, I am new to this forum and could really use some help. Not a beginner but a little rusty. I made a bucket hat size small for my grandson. His measurement is 18 1/4" so I made the small which said 19" finished size on the pattern. Pattern printed correctly with the 1" test square and I used the pattern seam allowances of 1/2". Well the hat is too small and the finished size it about 17", two inches too small. Does anyone have any suggestions? I want to be more careful with size going forward. Where do I measure the pattern piece for the hat to check the size? I would think it would be the top edge of the side piece 7c (less seam allowance) because this is the smallest edge that must fit around the childs head. Thank you for any help!