Professional critique: Not big enough, add more air.

But on a more serious note, the anatomy itself doesn't look too bad, but it might be a good idea to have the tail proceeding in the direction it's going out, especially since it's inflated and all that fun stuff.

Also, it'd be a good idea to not shade exclusively with black. Doing colored shading is a good way to make the style look more interesting. (I'm not absolutely certain that's what you're doing, but it looks that way, and it'd be better to play it safe and mention it anyway.)