Fourieranalysis in Proteus

Hi all, I am preparing something educational in Proteus.

I am driving a diode with a 1MHz signal and want to se the fourier spectrum.

I am prompted for a start time, a stop time, a max frequency and resolution as well as the different windows.

What is the connection between the stoptime and the resolution ?
If I choose a small time 1E-6 it says that it zeropads.

Could somebody explain some theory so that I can understand how to appreciate a suitable stoptime depending on the resolution I choose and the computing time I want to spend ?

Kindest regards,
StoppTidigare