Which actinoid from following in its +3 state has largest size ?

The actinoid series spans elements with atomic numbers 89 to 103, characterized by actinoid contraction—a steady reduction in atomic and ionic radii with increasing atomic number due to poor shielding by 5f electrons, enhancing effective nuclear charge.

Among the provided actinoids: Uranium (U, Z = 92), Berkelium (Bk, Z = 97), Einsteinium (Es, Z = 99), and Mendelevium (Md, Z = 101), the ionic radius in the +3 oxidation state diminishes from left to right. Uranium, with the smallest atomic number, must therefore exhibit the largest $\text{U}^{3+}$ ionic size.

Final answer: $\boxed{\text{A}}$