Ideally 30w per square foot is the aim. A 1.2m x 1.2m is 16 square feet so 480w is optimal.
1.5 x 1.5 is 25 square feet so 720w would give 30w per square foot.
So in a 1.2m x 1.2m either the ISH530 or 2 F bars will perfect at 480w / 30w per square foot.