FFT magnitude of ADC samples, converting back to $\textrm{dBm}$

1. What does the magnitude of an FFT mean?
2. What is length of signal in FFT?

What does the magnitude of an FFT mean?

Basically, the magnitude of the FFT is the amplitude of the associated frequency component. When you're using the FFT function in MATLAB you probably also want to use the fftshift function to center the results around 0.

What is length of signal in FFT?

The correct size of the FFT lengths for a linear convolution, is Nfft=Lx+Ly−1, where obviously those lengths correspond to the length of your signals x and y.