The main diagonal is 2. Because it is a cube, all the side lengths are the same, and we have the equation: \(\sqrt{x^2+x^2+x^2}=2\)
Solving, we find \(x^2 = {4 \over3}\). The surface area is \(6x^2\), meaning the surace area of the cube is \(8\).
Now, we have the equation: \(4\pi r=8\)
Diving both sides by 4, we get: \(\pi r = 2\)
Solving for r, we find \(r = \color{brown}\boxed{\sqrt{2\over \pi}}\)