History of the Cubit Rule

  Cubit is the name for any one of many units of measure used by various
ancient peoples. The natural cubit is based on the distance between
finger tip to the elbow on an average person. It was employed  to measure  cords and textiles.
  One of the earliest types of measurement concerned that of length.
These measurements were usually based on parts of the body. A well
documented example (the first) is the Egyptian cubit which was derived
from the length of the arm, from the elbow to the outstretched finger
tips.
  By 2500 BC this had been standardized in a royal master cubit made of black marble (about 52 cm). This cubit was divided into 28 digits (roughly a finger width) which could be further divided into fractional parts, the smallest of these being only just over a millimeter.

  There are many ancient cubits, ranging from a petite 17.5" to an outrageous 24", excluding even more radical candidates.  In the key civilizations like Egypt and Babylon the cubit had two distinct sizes, a shorter "common" cubit around 18" and a longer "royal" cubit of 20" or so.
  Let's assume for a moment that Noah used a long cubit, making the Ark
515 ft (157m) long.  Obviously the Ark should have been a perfect fit,
but what happens if those animals are now loaded into an Ark built instead with a cubit closer to 21"? They now have 60% more space.  To be a "perfect fit", either there were more animals, or the cages were bigger.      

 Stephen described Moses as "educated in all the wisdom of the Egyptians"
(Acts 7:22).  Moses would have known the royal and common cubit definitions from Egypt.  Throughout Genesis he uses the term "cubit",
but a contemporary measurement of the enormous bed of King Og is
qualified with the term "cubit of a man" , which itself sounds a bit
like "common cubit".  The giant Og, king of Bashan slept in a bed 9
cubits long.  By the short cubit (17.5") this is 13 feet, by the long
cubit almost 16 feet.  (Now that is excessive, making the short cubit
preferable).  

  Moses never made such a distinction in Genesis or Exodus, so this could be the first time he talked about the common cubit. Ezekiel measured the new temple with a royal cubit.   Regardless of whether people shrank or the royal cubit has always been "a cubit and a palm", God had Ezekiel use one of these to measure the temple. Not the ordinary cubit, the royal one.  The Hebrew craftsmen  should have been well versed in the royal cubit from Egypt before they built the Tabernacle.  In any case, the royal cubit is a natural choice for a project with religious significance.
  A stronger clue comes some time later when Solomon, following David's
divinely inspired directions  for the temple design, used "the cubit
after the first measure".  Which cubit was this? Obviously not the "usual"
cubit of the Hebrews, which looks very much like the common from Biblically like the Siloam tunnel .  So it must have been the royal, that Moses used for the Tabernacle and Ark of the Covenant.  This almost looks as though the royal cubit was the "correct" one for temples,  smething even the Egyptian pagans understood.
  Lastly, in Ezekiel's vision, an angel measures the temple with a reed
(rod) of 6 cubits, each cubit being of a "cubit plus a handbreadth".
Amazingly, some have argued against this being a definition of the
royal cubit, but to Ezekiel's audience , there is  probably no better way to say "Royal Cubit", since it was always one handbreadth longer than the common cubit - in both Egypt and Babylon.
  Short of the famous vessel turning up on a mountaintop someday,
establishing the exact cubit length used for Noah's Ark may appear
to be an impossible mission.  Pinpoint accuracy is unrealistic, but a
good place to start is simply this: Which class of cubit is the more
likely candidate, the "royal" or the "common"?

How big was Goliath?

So how tall was Goliath? That one is a little harder. In the story of David slaying the giant Goliath (I Sam., Chapter 17), the giant's height is described as "six cubits and a span." But nobody measures in cubits anymore.

A cubit is believed to be the distance from the elbow to the tip of the middle finger. A span is the distance from the thumb to the little finger with the hand stretched apart. But since arm and hand lengths vary, that doesn't give an accurate measure. And people are believed to have been much smaller thousands of years ago.

This is where archaeology may be a help. The Bible mentions Jehoash breaking down a 400-cubit section of Jerusalem's wall, from the Ephraim Gate to the Corner Gate (See II Kings 14:13). Another verse talks about two other gates being 1,000 cubits apart. If any of these gates can be found, the distance could be measured to get a better idea of the length of a cubit.

But there's another problem. In the Bible, numbers like 400 and 1,000 may not represent precise measures. They may be estimates. The length of the wall between two gates might have been 400 cubits exactly, but maybe 400 was just to give a general idea of its length. People have calculated Goliath's height at anywhere from 6 ft., 6 in., to 9 ft., 6 in. Others scholars say the numbers in the story are estimates - that all they mean is that Goliath was really big.