I recently visited San Diego to see the Dead Sea Scrolls, and there was a mistake where the scribe wrote that Goliath was six and a half feet tall, not nine and a half. I know since people were smaller back then, six and a half feet tall could easly be considered a giant. Still, I'm not sure. Do you think the older copies and the DSS were correct and all newer cpoies mistaken, or do you think the DSS really had this specific error?

Yes, it is an incredible exhibit, isn't (wasn't) it?  Anyway, copyists often made mistakes with numbers. The Hebrew text normally reads 9'4". That would be a real giant. But no, 6'6" would not be a giant! Take Saul -- he was was a head taller than everyone else (1 Samuel 9:2), so he himself was certainly over 6'.  But not a giant. I think the DSS in question had an error.