Not sure if there are exact measurements out there but this is the excerpt from LotR Weapons and Warefare.
During the Second Age, the men of the army of Gondor were great in stature; all were over six feet tall and very broad. A Gondorian shield was therefore long and broad and made of a number of different layers of wood, which were edged in metal and produce a strong, heavy protective item. The wood was stained almost black and the outer surface bore the device of the Tree of Gondor, also in metal, surmounted with the seven stars and crown. These were the royal emblems of the Gondorian people and showed thier noble Numenorean lineage. The shields were curved to help the soldier more easily deflect the blows of his enemy, and the bottom edge came to a point; as with the Elven shield, this reduced the weight of the shield and also meant that it could be rested or secured on the ground while being gripped.
I remember seeing a full size Weta replica at their booth at Armageddon (A con held in Auckland,NZ). I also know they sell a scale replica which has the dimensions of 9cm tall and 4cm wide, but I do not know what scale it is at.