Here, he’s shorter than the average foal, but longer in body. His wings are also still too small to be capable of anything more than short bursts of flight. Dragons also mature (mentally) differently from ponies; at eight years of age by the time Twilight comes to Ponyville, he has the mentality of a young adult (albeit with a childish streak), and was able to speak eloquently by the time he was only two.
It could be centuries before he grows to full size; however, he won’t end up very large.