Because the masses that you give are for blocks that are 1 cubic meter in volume, they also serve as the densities for the two metals that you are comparing.
mass = density*volume
volume = (4/3)*pi*r^3
volume of iron sphere = (4/3)*3.14*0.0201^3 = 3.40*10^-5 m^3
mass of iron sphere = 7860* 3.40*10^-5 m^3 = 0.27 kg = mass of Aluminum Sphere
Volume of Al Sphere = 0.27/2700 = 9.90*10^-5 m^3
Radius = cube root (volume / (4/3) / pi) = 2.87 cm.
I did this using the MS calculator, and I'm not 100% sure on the numerical answer, but the process is what you need to do to solve the problem. You should double check my answer.
hope this helped :)