Exerting control over a 5x5 square, especially when there are walls on two sides to cause chokepoints, is pretty easy.
The average male is around 5'8". The length from finger-tip to finger-tip is almost the same as your height. So the average knight standing in the center of a 5' box will be able to reach 4 inches beyond the edge of that box. A longsword has a blade of 3 to 3.5 feet, and a handle of at least 6". If we say that the last six inches of the arm and the first 6 inches of the sword overlap, then Bob the fighter has an effective reach of 5' in BOTH directions. He can easily exert control over the 2.5 feet on either side of him and it will certainly be difficult for anyone to get past him.
And, if you are using miniatures on 25mm bases, it is pretty easy to see how well a human sized creature can fill a 6x6 space.