Ichthyosaurs averaged about 2 â `` 4m ( 6 â `` 13ft ) in length ; a few were smaller , and some species grew much larger . They had a porpoise-like head and a long , toothed snout . Ichthyosaurs averaged 2 - in length ( although a few were smaller , and some species grew much larger ) , with a porpoise-like head and a long , toothed snout .