Stimuli are periodic, and there is control over their frequency and absolute phase. This a-priori information can increase the power of the response measures as compared to textbook examples on Fourier analysis. As we know the stimulus frequency exactly, we can avoid this artifact. Luckily, it doesn't affect the conclusion. The experimental parameters principally determine the number of data points per sweep.
Textbooks suggest to fill up with zeros.
This can be done with linear or non-linear interpolation. A very small amount of spurious frequencies will be generated that can be neglected against other noise sources. Typical flicker-ERG, PERG or VEP sweeps will have less than 1000 data points, so a numerical integration to calculate the Fourier coeffiecients will only take milliseconds on current computers. To calculate the entire spectrum, however, the calculation time will be noticable. The neighbouring figure shows that indeed many lines from the sawtooth spectrum are suppressed through windowing.