Which of the two have almost similar size

Nb and Ta have almost equal covalent and ionic radii due to lanthanide contraction. Lanthanide contraction is a term used  to describe the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, Lanthanum, to 71, Lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium. The effect results from poor shielding of nuclear charge (nuclear attractive force on electrons) by 4f electrons; the 6s electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius.