Under NFL rules, the height of goal posts must be at least 30 feet above the crossbar.
There is no limit on how tall they can be. In theory, they can be 500 feet tall and poke out of the top of the stadium, if some eccentric owner wanted them to.
But most NFL teams only meet that minimum 30-foot standard.
Every once in a while, a kick will go directly over the top of the goal post, and we'll have no idea whether it's good or not. There's no reason for it.
That's what happened today in the Broncos-Chargers game, when Matt Prater "made" a field goal attempt that went right over the post. It's impossible to tell whether or not it would have went in if the goal post was taller.
Right over the post (via @cjzero):
Close:
Eventually, this is going to happen in a really pivotal spot, and it's going to be ugly for the league.