I know of no skeletal evidence that gigantism was common among the Iron-Age Philistines. However, the very fact that Goliath's stature and strength was considered remarkable suggests that it wasn't common. If men of such stature were common, Goliath would not have been considered remarkable or unusual.

Furthermore, it's probably not a good idea to treat the Prophetic books of Samuel in which the original "David and Goliath" story is told, as straightforward historical accounts. They are clearly inspired by real events, but are basically mythological narratives that have been edited and tweaked to serve the interests of the Davidic dynasty that ruled over Judea and Samaria when the book itself was written. Modern scholarly analysis had been that the bulk of the books of Samuel was written in the early 7th century BCE (probably during the reign of King Josiah), over 200 years after the events it claims to depict. There are some further textual clues (like the use of a silver silver quarter-shekel coin) that suggest the current masoritic text was edited and redacted sometime in the Persian period, and the version we have today seems to date to sometime in the 5th or 6th century BCE.

If Goliath isn't an entirely fictional character, he probably was just an unusually tall and broad fellow of the sort that occurs occasionally in any human population.

This information is from my copy of Eerdmans Commentary on the Bible.

EDITS: Grammer, typos, etc.