Atomic radii decrease slightly across the 3d series due to poor shielding by d-electrons — each added d-electron shields less effectively than a p-electron, so effective nuclear charge increases gradually. The decrease is much smaller than in s- or p-blocks. Ti to Cu span only ~10 pm difference. At the end, radii increase slightly (Cu, Zn) due to electron-electron repulsion in the nearly/fully filled d-shell overcoming the nuclear charge increase. Ionic radii for $M^{2+}$ follow a similar trend with a dip at $Ni^{2+}$ and slight increase at $Cu^{2+}$, $Zn^{2+}$.