Length of DFT defines whether I see the harmonic or the dominant frequency
Zero padding does not increase the frequency resolution— it just interpolates more samples in frequency but does not give you any more information. The frequency resolution in Hz of an non-windowed (rectangular window) Fourier Transform is 1/T where T is the length of your time domain waveform in seconds. You must have a longer data sample in order to get higher resolution or see lower and lower frequencies. An extreme case is that you need an infinite amount of time in order to observe a true DC signal.

Note that zero padding gives you more samples of the Discrete Time Fourier Transform (The DTFT is the result if the time axis was allowed to go from minus infinity to plus infinity; by zero padding you approximate that). The Discrete Fourier Transform in contrast has a time axis this only goes from 0 to N-1.