With a 16' gate use 6" posts, buried 3' deep in a hole 12" diameter and fill with concrete. Also, if you can avoid it, don't use a corner post for your gate. While tempting, you need the ability to spread the load out into the other posts. Another trick that helps in the long run is to have a center support that both gate halves rest on. Also, keep your gate as light as possible. I would use metal straps for the joints.

Fred
52 3600
69 C-10