Calculating the Radius of a Molecule
When you divide the mass of the solid by its density then you get the volume of the object which means not only the volume of the molecules also the volume of voids. Now the volume you get is = volume of the particles + volume of voids. So the radius you get is = radius of void + radius of molecule.  So in order to solve the problem you must which kind of packing is this. And then you should know it's packing efficiency. So now the volume you got should be multiplied by the packing efficiency to get the actual volume of molecules. If you say why do they have voids. Then I'll  say you can't fill a particular shaped object with small sphere's so packing efficiency can't be 100%. So all you need to know is the kind of lattice formed by the molecules of amelopectin. And now what you got in your question is the volume of single particle + the volume of void. Well if you wanted to get the radius from it then it would be error as I said before. But in that case also you had to put $V=\frac{4}{3}\pi(r^3)$= the volume of the single molecule. And in this way you get the radius as 6.02*10^-8cm. And then get the radius. But actually taking the molecules as cube gives you a better answer though this would not give the proper answer if amilopectin molecules aren't cubic. But certainly you can fill a shape with cubes more efficiently (without having much voids) so in that case $a^3=volume you mentioned which is 9.17*10^-22 cubic cm. That gives a as 9.7*10^-8cm as the side length.