Tonker,
This is oversimplified, since other factors can add or detract a bit, but the most significant element of a ship's theoretical maximum speed is "hull speed", which is a factor of the square root of the hull length at or under the surface of the water. Others here can explain it better than I, but longer ships can go faster than shorter ships, all other things being equal. Going faster than hull speed involves huge amounts of power, not practical for most merchant ships.