The maximum packing density of spheres into a cube is π√2/6. This means that in a cube of side 80cm, volume 512000cc, 512000(π√2/6) cc is taken up by the spheres themselves. If 100 spheres are so packed, then the volume of any single one of them is 2560(π√2/3) cc. The volume of a sphere is 4πr3/3, where r is the radius, therefore 4πr3/3=2560(π√2/3), r3=640√2 cc, making r=9.67cm approx.