I love the length to this piece- long dimensions always make for really interesting compositions. I really like how chunky he is design wise, especially in the chest and shoulders area; He'd need massive muscles to be able to lift his bulk and I think you did well with that design aspect. The way the scales go along his spine really help the eye flow around the image too, which is great.
I know you're going to hate me for this, but I really think the tail is a massive disruption of flow. It would have to be massively long in order to curve around several times to end up in front of his head, and the top part above the moon is a little confusing as to where it's coming from. I'd imagine that if he was in the middle of breathing fire he'd be quite stiff and tense, and quite grounded, with his concentration focused on projecting the fire at his target. Therefore I'd imagine the rest of his body wouldn't be doing anything that might throw off his aim.
Removal of the tail above the moon would give him some really nice space too and help open the piece up a bit more.
I also think the tail really takes away from the lovely detail on his face. Another minor point is I think his face could do with more values and tones- I know there's a huge light source coming from his mouth, so there wouldn't be any crazy shadows, but I think some really bright highlights (nearly white) in certain places would really make it pop. Just my two cents!
All in all though, you know how much I love this piece. The contrast of the really dark rocks and trees work so well with how vibrant he is, and like I said earlier I love how you handled the gems/gold on his chest. I love how you did the fire too- it almost looks liquid or like molten lava, which is a brilliant effect. You can also really tell you've got to grips with digital art, the blending on everything is really nice, as are the textures on his scales (I expect nothing less from you in that area ) Brilliant work my friend, well done