For a four coordinate complex such as [Ni(CN)4]2- and [NiCl4]2-, we can decide its geometry by closely considering its magnetic properties. Both of the complexes are d8 complexes which could be found either in the tetrahedral or square planar crystal field depending on the nature of the ligand.
CN^- being a strong field ligand leads to the formation of a square planar diamagnetic d8 complex of Ni^2+. Similarly, Cl^- being a weak field ligand leads to the formation a a tetrahedral paramagnetic d8 complex of Ni^+ hence the answer given above.
The atomic number increases moving left to right across a period and subsequently so does the effective nuclear charge. Therefore, moving left to right across a period the nucleus has a greater pull on the outer electrons and the atomic radii decreases.
