He figured out a way to objectively measure the height of this Ruby figure.
It starts by recognizing that he was about as tall as the top of that elevator panel to his right, our left.
That elevator got taken out, but the panel is still there. Here is Detective Elmer Boyd standing next to it in 2003. He was probably there for some 40th anniversary gathering.
So, the panel was at the level of his nose. So, what was his height? He was taller than Oswald.
So, that means that the Ruby impostor was as tall as the line you see across Oswald's brow. And WWW estimates that to be about 3 inches shorter than Oswald. And since Oswald was 5'9", it makes the Ruby impostor 5'6".
Keep in mind that this process works; it is objective; and it is independent of knowing the man's identity. No matter who he is, regardless of who he is, he is 5'6". But, Jack Ruby was 5'8".
And we knew all long, just intuitively, just from looking at him, that he was too short to be Jack Ruby.