Appearance from his Chapter 81 introduction through Chapter 460.Even amongst the shinigami, Hitsugaya is noted to have an unusual appearance. Before he became a shinigami, he was ousted in his home district in Rukongai; one of the reasons cited is his appearance. He has messy, stark white hair and unusual blue-green/teal colored eyes. As the Tenth Division Captain, he wears his shoulder-length white hair slicked back, though a tuft of hair still falls to the left side of his face. He wears the traditional black shinigami garb, overlaid with the prestigious white Captain's haori. He wears his sword tied behind him, held in place with a blue-green sash with a bronze metal adornment in the center. The sash's color matches his eyes in manga colored illustrations.
It is interesting to note that Hitsugaya is absolutely tiny. It can be difficult to see sometimes with Tite Kubo's (gorgeous) art style, but Hitsugaya measures at only 4'4" and weighs about 62 lbs. To get a better idea of his size in our world, I did a bit of research. According to the average height/weight growth chart in adolescents across the globe, this means that he has the same stature as a young boy within the range of eight to twelve years old. He is literally the size of my nine-year-old niece. He is so small that he can't wear his sword at his belt because it would drag on the ground, which is why he must wear it strapped to his back. This small frame is what makes allies treat him like a child, and enemies belittle and underestimate him. It... seems to be a sore spot for him.
Appearance after the Chapter 460 time skip through Chapter 686.Over halfway through the series, there is a time skip of about a year and a half, where many of the characters change their appearances to varying degrees. After the time skip, he is shown to wear his hair loose, rather than his previous slicked back style. His hair is kind of messy even when styled back, so I can only assume that he kind of... gave up on styling it everyday. Either way, I think it is an improvement. This style must be the way his hair falls naturally, because it is nearly identical to when he was younger, though a bit longer than it was. It could very well be that he is simply more relaxed after the conclusion of the events with Aizen and his appearance reflects his change in attitude.
He still wears the standard shinigami kimono and haori over it. It doesn't seem as if the poor Captain has grown any taller in the year and a half that passed, so he still wears his sword strapped to his back. It is held in place with a bronze chain, with the same adornment on it. The sash he used to tie it back with seems to have become somewhat tattered over the time passed, and he wears it as a scarf instead. The significance of this sash and why it became so tattered are never explained, but I imagine it must be important to him if he still wears it as it is. Perhaps it was a gift from his grandmother or Hinamori?
Appearance before the story events in the prequel Chapter -16, when he was younger.Believe it or not, there was once a time when Hitsugaya was indeed smaller than he is now. His height and weight aren't specified in the prequel chapters, though it is safe to say that he has always grown at a rather slow rate. In the five years that pass in his prequel chapter, he states that he hasn't grown at all. The Hitsugaya of Chapter 460 and beyond looks very much like the Hitsugaya seen in this particular chapter, though his hair was somewhat shorter when he was younger, and seen parted in the middle rather than to the side. Without the flowing shinigami kimono and oversized haori, it is much easier to see just how tiny he really is. Aside from the length and parting of his hair, he hasn't changed much since his pre-shinigami days.
Temporary aged appearance revealed in Chapter 670.If anyone was wondering what Hitsugaya would look like as an adult, it seems your questions have been answered. Within the last chapters of the series, Tite Kubo unveils Hitsugaya's adult form. In the story, this form is temporary, as his bankai changes his form in order for him to better wield its power. The explanation is iffy at best, but... adult Hitsugaya is damn sexy. I mean, seriously. It seems that one day, he will eventually hit that growth spurt he's been waiting forever for. He might even be able to wear Hyourinmaru at his side instead of his back! Good for him! :'D
When accelerating his growth, his clothing doesn't change, unless you count it shredding because it doesn't quite fit anymore as changing. As an adult, Hitsugaya is thin and lanky, and his features become much sharper. He still has his big half-lidded grumpy eyes and sour expression, and his hair hasn't seemed to calm down its unruliness with age. I find it hilarious that after all the growing he wants to do, when he unveils his adult appearance, he says he isn't fond of it. Fickle, dear? :) He has a long time before he actually looks like that though, so he'll have time to get used to it. In the series' final chapter, we see that he has reverted to his normal size, proving that his form was temporary and brought on by his bankai.