My brother was involved in the repatriation act, working with Douglas Owsley. I asked him once about giants, and he just laughed and told me that he had not seen any. But I did not ask him if he considered a 7" tall human being to be a giant. Dr. Greg Little wrote a recent book about Native Americans and the constellation of Cygnus. Though it was primarily a book about Native American religion, he did some research into giants in America, concluding that 7' tall or a little more, was about the extent of height that any Pre-columbian people reached.
However, when you go back to the most recent ice age you had oversized sloths, buffalo, wolves, lions, etc... so why not oversized people? The big question is why were they oversized? Was the moon closer to the Earth at the time, decreasing the Earths gravitational effects? Diet? If there were giants, as so many ancient cultures have indicated, what would cause this "largeness"?