Among the III A group elements, the difference in the atomic radius is large in between

B: Boron has the smallest atomic radius among group 13 elements.

Ga: In Ga, the d orbital does not shield the nucleus effectively. So the effective nuclear charge is higher for the outermost electrons. So, the atomic radius is smaller.

Al: Down the group, the effective nuclear charge decreases as the number of shells increases, So the atomic radius of Al is larger.

In: In In, the d orbital does not shield the nucleus effectively. So the effective nuclear charge is higher for the outermost electrons. But down the group, the effective nuclear charge decreases as the number of shells increases. So, these two effects result in the atomic radius of In similar to the atomic radius of Al.

So, the Atomic radius order is: B < Ga < Al ∼ In.