This article may be relevant: J. Daniel Hays, "Reconsidering the Height of Goliath," Journal of the Evangelical Theological Society 48, no. 4 (2005): 701-14 [link].

Hays points out that earlier witnesses to the current text of 1 Samuel 17:4 (i.e., major recensions of the Septuagint, Josephus, and a Hebrew manuscript from Qumran) list Goliath's height as "four cubits and a span," about 6'9", rather than "six cubits and a span," or 9'9". The discrepancy may have resulted from parablepsis, though Hays' explanation (on p. 706) does seem convincing to me. At any rate, an original height of 6'9" would be more realistic; of course, this does not prove in any way the historicity of the tale.

Hays also criticizes the gigantism theory, noting that "one of the symptoms of gigantism is not only poor eyesight (esp. peripheral vision) but also overall physical weakness... Was Goliath tall, skinny, clumsy, weak, and nearly blind? Hardly a formidable foe!" As a stronger parallel, Hays cites Andre the Giant, "who was 7'4", weighed 500 pounds, and was agile enough to participate in professional wrestling (WWF)" (710 n21).

In a response to this article, Clyde E. Billington ("Goliath and the Exodus Giants: How Tall Were They?," Journal of the Evangelical Theological Society 50, no. 3 [2007]: 489-508; [link]) suggests (among other things) that giants really did exist historically in the region and that the differences between the texts reflect different definitions of a cubit. Personally, I find his arguments difficult to take seriously. For Hays' rebuttal, see in the same issue "The Height of Goliath: A Response to Clyde Billington," 509-16 [link].