Our quantum mechanical description of an atom suggests a very broad region for finding the electron. It is difficult to define a sharp boundary for distance between the electrons in any particular atom and the nucleus. So some approximations are made in determining this parameter.
Empirically measured atomic radii
After collecting large amounts of data generally accepted atomic radii are known for most elements in the periodic table. Atomic radius decreases from left to right across a period. It is believed this is due to stronger attractive forces in atoms (as you go from left to right) between the opposite charges in the nucleus and electron cloud cause the atom to be 'sucked' together a little tighter. Atomic radius increases as you go down a group. There is typically a significant jump in the size of the nucleus (protons + neutrons) each time you move from period to period down a group.