What is a Cubit?

Hebrew Short: 17.5 in
Egyptian: 17.6in
Common: 18in
Babylonian Royal: 19.8in
Hebrew Long: 20.4in
Egyptian Royal: 20.6in

A cubit was considered the length of a man’s arm from his elbow to the tip of his fingers. OK, whose arm are we going to use? It would be easy to see that depending on the man, the length would be different. Even at that, there were some “standards”. Generally there are two lengths that were common. There was a short cubit at 18” and a long cubit running 20”. For the making of this spear we tried to always go with the most conservative weights and measurements knowing that people are going to have a hard time believing this thing was as big as it is. The truth is, it may have been even bigger. We went with the shorter, 18” cubit.

https://answersingenesis.org/noahs-ark/how-long-was-the-original-cubit/

So what is a span?

A span is the distance between the tip of the little finger and the thumb when you fully extend your hand. But your hand may not be as big as mine or mine may be a lot smaller than yours. So again, whose hand do we use?

We chosen to use the most conservative length of 6 inches.

https://answersingenesis.org/bible-characters/david-little-guy-or-mighty-man-of-war/