As our YouTube video demonstrates, not all posts for decking and outdoor porches are created equal. A general rule of thumb is that if a deck is higher than 24 inches off the ground, the posts that support it should have some heft — that is, some weight or bulk that will prevent the deck from looking awkward. A high deck on 4 x 4 posts would tend to look flimsy. That’s why we use 6 x 6 lumber for deck posts.