The size of Nb is very similar to

The size of Nb is similar to Ta due to the lanthanide contraction.

The similarity in the properties of the metals depend upon the size of the metals. The size of the metal depends upon the effective nuclear charge on the outermost shell.

In lanthanides, the electrons also fill in the f -orbital and the shielding power of the f-orbital is very less. So, the attraction of the nucleus on the outermost shell increases thus the size of the metal decreases.
So, the reason for the similarity in between niobium and tantalum is the size which is the result of the lanthanide contraction.
Thus, the niobium and tantalum have the same size and due to the same size, tantalum and niobium have the same ionic and covalent radii.