The expected trend would be (as you go down a group):
First row < Second row < third row
This is the case for groups one and two (before the d-block), for the transition metals the first row is the smallest in radii, but the second and third rows are almost identical in size. (In group three the third row element is actually smaller then it's second row counterpart.) Why does this occur?
It is due to the Lanthanide contraction. The elements in the third row of the d-block group of elements are preceded by the Lanthanides / Lanthanoids (which are the first row of the f-block metals group) (seen later in the website) therefore the third row d-block elements will contain f-orbitals. F-orbitals have very poor shielding properties. The repulsion between the adding electrons and the the precedong orbitals is not substantial enough to outweigh the attractive forces from the "deshielded" inner nucleus, so the outer electrons become more drawn in and the atoms become more compact.
Author: Sally Robinson, date of modification: 20 March 2003