The base of our post caps slide down the post 1.5", except on the Economy models, which only slide down 3/4". At their widest points (corner to corner), our post caps are as follows: 6x6 Post Caps (Nominal and Actual): 7.5" widest point 4x4 Post Caps (Nominal and Actual): 5.5" widest point Economy Post Caps (Nominal only): 4.5" widest point