In the book, Stephenie Meyer does not give a detailed description of Bella, so that people can get in her shoes better. However, on her website she has included a fuller description:
In my head, Bella is very fair-skinned, with long, straight, dark brown hair and chocolate brown eyes. Her face is heart-shaped—a wide forehead with a widow's peak, large, wide-spaced eyes, prominent cheekbones, and then a thin nose and a narrow jaw with a pointed chin. Her lips are a little out of proportion, a bit too full for her jaw line. Her eyebrows are darker than her hair and more straight than they are arched. She's five foot four inches tall, slender but not at all muscular, and weighs about 115 pounds. She has stubby fingernails because she has a nervous habit of biting them.
In the movie version of the book, Bella was played by Kristen Stewart.