Power Spectrum Density and Frequency
If you are asking about the grid resolution of the PSD then it is given by:

$$ \frac{{F}_{s}}{N} $$

Where $ {F}_{s} $ is the sampling rate and $ N $ is the number of samples.
Pay attention that you can increase the grid resolution by Padding the data with zeros.
Though it will increase the grid resolution it won't increase the resolution of the DFT (In the sense of resolution to see to close, in frequency, harmonic signals).