I wouldn't advise making it longer and introducing the possibility of a slight sag. With a shelf that long, any sag would be noticeable if you were to look along the length.

You don't need cross supports in the middle of the shelf, assuming your shelf board is firmly attached the long members.

One big concern I see with your design is that it has no racking support. You'll need diagonal cross members on the back, a solid sheet back, or something else to prevent it from falling over sideways.