Final answer:
The trend in atomic size in the transition series is influenced by the effective nuclear charge and the shielding effect, causing the atomic size to first decrease, then stabilize, and eventually increase.
Step-by-step explanation:
The size of atoms in the transition series first decreases, then becomes constant, and then increases because of the effective nuclear charge and shielding effect. Initially, as we move from one element to the next across a period, the nuclear charge (Z) increases by one, but the shielding by core electrons increases only slightly. This causes the effective nuclear charge (Zeff) to increase, pulling the electrons closer to the nucleus and making the atomic radius smaller.
However, as we add more electrons in the d-orbitals, they start to experience electron-electron repulsions which can counteract the increase in Zeff. This is why the size becomes relatively constant in the middle of the series. Towards the end of the series, the atomic size can begin to increase due to added electron-electron repulsions and the addition of more electron shells as new sub-levels are being filled.