As we move from left to right in a series from Sc to Mn atomic radii decreases. This decrease occurs because of increase in nuclear charge that occurs. After Mn, electron pairing occurs so shielding effect of d orbitals increases which try to increase the size. From Fe to Ni size almost remains constant because of two factors playing role simultaneously i.e nuclear charge effect as well as shielding effect. After Ni, size increases from Cu to Zn, because Cu and Zn have completely filled d orbital so shielding effect plays dominant role.