How big of a hutch does the average rabbit need?

The size of hutch you will need is going to depend a lot on the breed of rabbit that you are getting. If it is a little Dutch Bunny, she will only be a little larger than a guinea pig when she is full grown, however if it is a French Lop, she may grow to 15 lbs or more. This is going to make a difference on where you can keep her, as well.
If she is small, and you want to, she can be an inside rabbit. However, if she is a large lop, or other large breed, then an outside cage is going to work a lot better for her, so she can have more room to move around, and can see outside.
If you can also get one of those round portable dog pens with the little gate, then you can let her out in the yard to hop around and eat grass in the nice weather.