The atomic radius of which element is least

Atomic radius order :Tl~In >Al > Ga > B

                           OR

                         B < Ga < Al < In ~ Tl

From B to Al atomic radii increases as usual because, as the number of shells increases size increases.

From Al to Ga atomic radii should increase but here Al to Ga atomic radii decreases.

Reason,  Ga [Ar] 3d¹⁰  4s² 4p¹

Due to the presence of completely filled inner shell 3d¹⁰  electronic configuration .The poor shielding effect by innershell d-orbitals electrons ,the valency electrons are strongly attracted by nucleus and hence the size decreases from from Al to Ga.

"In,Tl"have almost all same atomic radii due to poor shielding effect by innershell f-electrons and d-electrons the valency shell electrons in "Tl" are strongly attracted by nucleus and hence the sizes of "In" and " Tl " are almost all same.