Every halogen atom has an average atomic radius determined by the attractive forces between protons and electrons, the repulsive forces between particles of the same charge. The halogens in higher periods have greater atomic radii because there are more energy levels' worth of electrons in the atoms with more protons.
Thus the atomic radii of the halogens is, in increasing order:
Fluorine, Chlorine, Bromine, Iodine, Asatine
This same pattern holds for all of the groups in the periodic table.
But what about the case where a halogen atom acquires an additional electron and a full octet? Well, that is one more electron that is thrown into the mix in the heap of repulsive forces occuring between all of the electrons. Thus more space is required for all of the electrons to exist in the same ion, so they spread out, thus increasing the atomic radius of the atom as it becomes a halide ion.
Compared to the noble gas atoms, the halide ions have slightly bigger radii because the noble gas atoms have one more proton than the halide ions in the same period. Thus the electrons in the noble gas atoms are drawn slightly closer to the nucleus despite repulsive forces between the electrons. For example, the atomic radius of a neon atom is slightly smaller than the ionic radius of the fluoride ion (\(F^{-}\)). The atomic radius of an argon atom is slightly smaller than the ionic radius of the chloride ion, and so on.