Well, any reader picking up this story and wondering what the title refers to will very quickly have their question answered! Kafka tells his readers in the very first paragraph what "metamorphosis" he is referring to by introducing us to his protagonist, Gregor Samsa, waking up in his bed from "anxious dreams" only to find that he has been transfigured into an enormous (to use Kafka's word, "monstrous") bug.
We can assume, from the description in the first paragraph, that the bug is approximately the same size as a human, given that Gregor is still in his bed, although the blanket is about to slide off him. The bug is possessed of an "armor-hard" back and has an arch-shaped abdomen, which is divided into stiff sections. The legs of the bug are extremely thin in relation to the rest of its body.
We can also tell, given that we are seeing all this from Gregor's perspective, that the transformation has been only physical; Gregor is still able to think, to be anxious (justifiably so), and to wonder what on earth has happened to him.