Comment by Symmetry

The formula for the equilibrium temperature for a sphere in sunlight is

    2 * pi * r^2 * L / (4 * pi * d) * (1 -a) = 4 * pi * r^2 * sigma * T^4

As you can see there are pi*r^2 on both sides of the equation, the surface area to cross section ratio of a sphere doesn't change as it gets bigger and so the equilibrium temperature doesn't change no matter how big the sphere is. (d is the distance to the Sun, nothing to do with the sphere itself).