About frequency resolution
The longer the total sample window (T0), the smaller the frequency difference between two sinusoids with exactly M periods and exactly M+1 periods (for integer M &lt;= N/2) in that window, which is required for orthogonality between adjacent DFT bins.

Also, the bin spacing is not the resolution. The frequency resolution can be greater or less than the DFT bin spacing, depending on the S/N ratio and your resolution needs (separation or estimation).