From an article in the New Scientist on how satellites happened to measure the open-ocean wave height (50 cm!) of the the recent Indian Ocean tsunamis:
The satellites saw the first two wavefronts produced by the main quake, spaced 500 to 800 kilometres apart. These reached a maximum height of 50 centimetres in the open ocean.
By comparison, wind-driven waves typically reach 10 metres. However, these waves only involve water close to the ocean surface and are separated by at most several hundred metres.
Tsunami waves have a very deep reach indeed – 4000 metres or more. “These waves involve the entire depth of the ocean. Their energy is thousands of times more than a monster storm,” Fu says.