Appearances in the Novels
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
The Valyrian greatsword Ice is described as taller than an adolescent Robb Stark. Based on this, an "educated guess" is that the sword is about 1.65 meters long (or 5 feet and 5 inches). However, as Valyrian steel is considerably lighter than common steel, Valyrian greatswords are likely larger in general.
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).