Answer
I'm afraid I don't know many theories about Goliath. Part of the problem is the discrepancy in his height. Some manuscripts list him as being 4 cubits and a span, some list him as 5 cubits and a span, and others as 6 cubits and a span. Thus, he was either 6 feet and a few inches (2 meters), almost 8 feet (nearly 2 1/2 meters), or 9 feet and a few inches (almost 3 meters) tall. 4 cubits and a span is big, but not terribly remarkable. Since Goliath is depicted as rather huge, this may well be wrong. 5 meters and a span is rather formidable, and highly unusual (it would make him extremely tall even for a professional basketball player in the NBA). 6 meters and a span, by today's science, would probably be considered a genetic mutation.
The Bible itself offers no explanation of Goliath's size, and also provides no information that any "race of giants" ever existed, except for the debated issues of the "rephaim" and "nephilim." These terms are sometimes thought to indicate races of giants, but I don't think these are the best explanations of the terms. In any event, Goliath is not said to have been one of the rephaim or one of the nephilim.
"Nephilim" was probably just a generic term for extremely ferocious warriors. "Rephaim" may have been a name for particularly large people (e.g. Deut. 2:20-21), or even a name of people who lived in the locale of Rephaim. It may also mean "ghosts," so the references to the large Rephaim may have more to do with the fact that they were only formerly powerful, or that little was known about them. The Anakim also seem to have been tall, but we have no good indication that they were giants. My own speculations are simply that some people are genetically bigger than others, and that as a result some people groups are a bit larger. Also, some individuals, either through normal genetics or through mutation, become exceptionally large. There is no indication in Scripture that any of these people were anything but fully human.
Answer by Ra McLaughlin
Ra McLaughlin is Vice President of Finance and Administration at Third Millennium Ministries.