Sesshoumaru's true form is that of a huge, white, fluffy dog (giving him his nickname, "Fluffy" ^^). He first transforms in the episode "Inuyasha vs. Sesshoumaru", while, obviously, he was fighting Inuyasha.
Fluffy-sama (as I call the doggie form) is at least twenty feet tall and covered in fluffy white fur. He still has the purple crescent-moon on his forhead, the purple stripes (though they curl around his mouth), and the furry "thing" he always carries over his right shoulder.
While in this form, he usually attacks using his paws or teeth, or stomps on people. But his saliva is toxic in this form, and melts anything it touches almost instantly.