I forgot to mention earlier, we use concrete as long as it's a permanent
sign. One bag for 4 x 4 posts and 2 bags for 6 x 6 posts. Dump the concrete in around the post and back fill with dirt. Let the ground moisture take care of the rest.
We have red clay and strong dirt. But I still don't see how 6X6's look better. Are you building an entire framing system? or just using brackets and putting between the posts?
Personally, I just think bigger posts look better and I'd imagine others here would agree with that as well. For one thing, it gives the overall
sign more dimension and again I'd rather have
signs that don't look like every other generic
sign out there on 4 x 4 posts. You can do a lot more with 6 x 6 posts, like add dimensional trim and moldings and have a wider selection of finials. 4 x 4's are okay for some things like smaller
signs and temporary
signs, but for a 4 x 8, I'd recommend 6 x 6 posts all day every day just for aesthetics, let alone the fact that they are much stronger. We seldom use brackets between posts. We either use cross bars, or a framework that fits the entire
sign. Occasionally, we've also been known to sandwich 3/4" painted MDO between two 1/8" ACM panels and slot the posts to fit the panel.