Wider does flow more water, but it's now known that the size of the screen is more important so that it can grow thick and green which gives the most filtering.
Why not cut the last 6" off the ends of the screen off and seal the slot with tape. Then cut the screen to 4" high. This will be 4 X 36 = 144 sq in., which needs 144 total watts, and will handle 12 cubes of food. If you make it shorter than 4", it won't catch all the light. As it is, you'll want the bulb within an inch of the screen. Should be ok for 8 cubes.