"And there went out a champion out of the camp of the Philistines, named Goliath, of Gath, whose height was six cubits and a span." (I Sam. 17: 4)
According to Fausset's Bible Dictionary, we learn the following about the Hebrew cubit.
MEASURES: Those of length are derived from the human body. The Hebrew used the forearm as the "cubit," but not the "foot." The Egyptian terms hin, 'ephah, and 'ammah (cubit) favor the view that the Hebrew derived their measures from Egypt. The similarity of the Hebrew to the Athenian scales for liquids makes it likely that both came from the one origin, namely, Egypt. Piazzi Smyth observes the sacred cubit of the Jews, 25 inches (to which Sir Isaac Newton's calculation closely approximates), is represented in the great pyramid, 2500 B.C.; in contrast to the ordinary standard cubits, from 18 to 21 inches, the Egyptian one which Israel had to use in Egypt. The 25-inch cubit measure is better than any other in its superior earth-axis commensurability. The inch is the real unit of British linear measure: 25 such inches (increased on the present parliamentary inch by one thousandth) was Israel's sacred cubit; 1.00099 of an English inch makes one pyramid inch; the earlier English inch was still closer to the pyramid inch. (Fausset's Bible Dictionary - see here)
Thus, a "cubit" is 25 inches (25.01), and a "span" is half a cubit. Those who think that the Jewish cubit was only 18 inches are wrong and have no proof for such a view. The evidence for a 25 inch cubit, however, has all kinds of evidence.
Given this information, we may say that Goliath was 172.5 inches, or about 14 feet and 4.5 inches.