The size of football goal posts varies with the level of football. While the height of the crossbar is fairly universal, the width between the two posts and the height of the posts above the cross bar are what differ.
In the NFL, the crossbar is located 10 feet off the ground. The goal posts are 18 feet 6 inches apart. This is common for most other outdoor professional leagues. The most prominent indoor football league, the Arena Football League, uses posts that are only 9 feet apart and a cross bar that is 15 feet above the playing surface.
Similar to the NFL, the NCAA uses goal posts with the same dimensions for both the distance from ground to the cross bar and the between the two posts. At the high school level, the goal posts are 23 feet 4 inches apart.
The height of the posts above the cross bar also varies by level. While this does not affect the degree of difficulty for kicking the ball through the posts, it can affect the official who verifies if the kick is successful. For example, a close kick that would hit the upright on a taller post would pass over a shorter one, leaving the call up to the official's interpretation.