The correct order of size would be :

As we know Pt is 6th period element, Pd is 5th period element and Ni is 4th period element.

The correct order of size is,$\mathrm{Ni} < \mathrm{Pd} < \mathrm{Pt}$

Because going downward in the group there is increment in the size increases as well as the addition of one more shell.