Does the electron cloud determine the size of the atom?

More or less, but all atoms are about the same size as each other. You would think that heavy atoms, with many electrons, would be much larger, but they aren’t. The reason is that large atoms have large nuclei with many protons. These exert a strong force on the
electrons, pulling them in closer. So even though there are more electrons in large atoms, they are
pulled in closer, leaving the overall size of the heavy atoms the same as for light ones.