The champions of the northern mountain clans wield huge two-handed greatswords. Greatjon Umber is also described as using one. Household troops of the Dreadfort wield greatswords along with other weaponry.
Speculation about size
A good estimate for the size of a typical greatsword would be 1.5 meters (about 5 feet). The size of the sword depends on the person wielding it though, so swords wielded by exceptionally large men may even exceed 1.8 meters (roughly 6 feet).