What periodic trends of Atomic size occur with the halogens?
The halogen atoms increase in size down the Group.
Now it is well known that atomic size decreases across a Period, and increases down a Group. As we go across the Period from left to right, nuclear charge increases sequentially, and the added electrons do not shield each other effectively. The result is a contraction ACROSS the Period. Going DOWN the Group, the added electrons have completed a shell, and thus offer some shielding of the nuclear charge; i.e. the valence electrons build on a shell of electrons that are already present. Shielding is the dominant effect, and atomic radii should reasonably increase.
As chemists, as physical scientists, we should put some numbers to our argument. See this site for actual numbers.