The selected FFT size directly affects the resolution of the resulting spectra. The number of spectral lines is always 1/2 of the selected FFT size. Thus a 1024 point FFT produces 512 output spectral lines.
The frequency resolution of each spectral line is equal to the Sampling Rate divided by the FFT size. For instance, if the FFT size is 1024 and the Sampling Rate is 8192, the resolution of each spectral line will be:
8192 / 1024 = 8 Hz.
Larger FFT sizes provide higher spectral resolution but take longer to compute.
The current FFT size is always displayed in the status bar along the bottom of the application for the active channel. The active channel corresponds to the plot which has the "focus" (its title bar is highlighted).
See also: Sampling Rate