  We study the thickness dependence of the electronic, magnetic, and optical
properties of a NiPS$_3$ thin film, an antiferromagnetic charge-transfer
insulator. Utilizing state-of-the-art advanced density functionals, we find the
antiferromagnetic Zig-Zag order, the band gap, and the main peaks in the
dielectric tensor are all in good agreement with the corresponding experimental
values. Upon thinning, the Zig-Zag antiferromagnetic order becomes virtually
degenerate with a competing N\'eel order, consistent with the suppression of
long-range order observed by Raman spectroscopy due to strong magnetic
fluctuations. Additionally, due to the robustness of the electronic band gap
observed by spectroscopic ellipsometry measurements above $T_N$, we suggest
that the persistence of the band gap is driven by strong electronic
correlations. Other systematic changes in electronic dispersion, effective
mass, and Kerr angle with thickness are also discussed. Finally, an applied
external electric field is found to suppress the band gap by up to 13%, until
precipitating an insulator-metal transition at a critical field value of $0.7$
eV/\r{A}.
