What is the ionic radii trend?
It decreases across a period and increases down the groups.
As you go across a period in the periodic table, the atomic charge increases, and this pulls the electrons closer into the nucleus, therefore decreasing the atomic radius. There is a little bit of extra shielding from sub levels, but this is outweighed by the increased atomic charge.
Down the groups of the periodic table, each new group represents another energy level being filled, which means no only are the electrons in a higher energy level, but also that there is a fair bit more shielding. Both of these factors outweigh the increased nuclear charge.