what is "forty cubits" and a "span"?

Answer 1

A cubit is about 18 inches, the average distance from the elbow to the finger tips. A span is the same as the width of a person's hand. So multipy 40 by 18 and add 4 and you will get an approximation of the number of inches.

Answer 2

I always hate people that cut and paste, but here goes. I have known for a long time a cubit is the length from finger tip to elbow but the unc site explains it better.
cubit
a historic unit of distance frequently mentioned in the Bible. The word comes from the Latin cubitum, "elbow," because the unit represents the length of a man's forearm from his elbow to the tip of his outstretched middle finger. This distance tends to be about 18 inches or roughly 45 centimeters. In ancient times, the cubit was usually defined to equal 24 digits or 6 palms. The Egyptian royal or "long" cubit, however, was equal to 28 digits or 7 palms. In the English system, the digit is conventionally identified as 3/4 inch; this makes the ordinary cubit exactly 18 inches (45.72 centimeters). The Roman cubit was shorter, about 44.4 centimeters (17.5 inches). The ordinary Egyptian cubit was just under 45 centimeters, and most authorities estimate the royal cubit at about 52.35 centimeters (20.61 inches).

And, I admit, I did not know span, but here's another paste:
span
a traditional unit of distance equal to 9 inches (approximately 22.9 centimeters) or 1/4 yard. This distance represents the span of a man's hand with fingers stretched out as far as possible. The Old English word spann meant precisely this unit of measure; all the other uses of the word "span" came later.

Answer 3

a cubit, if im not mistaken, is an old term for the length between the tip of your finger and your elbow.

A standard distance has been agreed on, i think, but im not positive what it is