The giraffe is the world’s tallest land mammal. They can reach heights of seven feet with their neck alone; with their whole body, they can reach a total height of twenty feet for a male, and fifteen feet for a female. That’s a full grown animal, by the way. A cal can easily reach two or three feet tall.
The calves are the only ones that sleep in a sitting position; the adults are too tall to make it feasible, so they sleep standing up, and may sometimes keep one eye open to look for danger. They can easily outrun any predators that try to come at them, but they have to get up to a running speed to do that. If they’re caught lying down on the job (literally), they can’t do that.