Appearance
According to the BSSM Materials Collection, Hotaru reportedly has fair skin and is very delicate and weak. (This should be obvious due to her existence as a cyborg.) In the manga, her eyes are black, but in the anime, they appear to be purple. I should think it odd to have a character with completely black eyes, especially in anime, which is probably why it was changed. Also, her voice is supposed to be low and quiet.
Her hair is black and fairly short, though longer than Ami's. I think this was meant to be a direct contrast with Usagi's hair - Hotaru's hair is short, while Usagi's is long; Hotaru's hair is dark, while Usagi's is light-colored; and Hotaru doesn't do anything special with her hair, whereas Usagi puts it up. This might be looking a little too deep into hair, but as they are the opposing messiahs, it's certainly plausible.
Height-wise, with heels on, Hotaru is a few inches shorter than Michiru and goes to about her eye level. Of course, this is before she is reborn - afterwards, she is quite a bit younger, and therefore quite a bit shorter. Then, she is about Chibiusa's height.
That aside, Hotaru is considered to be fairly beautiful. Although perhaps not quite as gorgeous as Michiru, Hotaru is a little more than just "cute."
Before her rebirth, Hotaru usually wears black clothing that covers her body. Even in summer she wears tights and long sleeves, due to the fact that her body is part machine. We get to see her in the Infinity Academy uniform, sporting the ubiquitous blue bow of the middle school division. At home, she wears black turtlenecks with black skirts and black tights. A lot of black, but she isn't what you would call "goth."
After her rebirth, Hotaru wears brighter colors and whatever Haruka, Michiru, and Setsuna pick out for her.