You could definitely make it bigger and would cover more overall squares if you did. So let's say we use the full 20' diameter. You would exclude 9 squares in the middle and then it would go 3 squares out form there. So it would cover a total of 72 squares, 9x9 for 81 less the 9 excluded.

The small radius just excludes the fewest number of squares in the middle, but especially if it's around you and you're not in melee yet, a larger circle could be useful. Just be careful because that could give the enemy a safe square to stand in and attack you. With only one safe square, you occupy it and anyone trying to attack you has to stand in the wall.

I just looked up AOE placement on different forums and some people require it to start at a grid line intersection and exclude squares in the corners, which would change my math, but the overall idea is the same.

I think the thing to keep in mind for wall of fire is if you make the circle up to a 20' diameter and then point it out, it covers a pretty good area.