A Great Sword is a sword made with:

3 Ingots

Two Hand Guard

Two Hand Grip

Heat the Ingots and put them on the anvil. Hold down 'Z' while looking at the Anvil and select 'Blades' from the radial menu. Hit the heated ingots until they turn into a large sword blade. To change the design just hit it until you are happy with the design then dip the blade in the oil and bring it to the workbench to connect to the guard and grip.