Obtaining real numbers from FFT with same length as original signal
Since you are reproducing the paper's results, read it carefully.
At the bottom of page 4 it says

This representation is formed by complex numbers, eliminating the imaginary part of each number in the frequency-domain signal. For this transformation, it is needed to calculate the power spectral density (PSD), as shown in Equation (3),
$$
P = \lim_{T\to\infty} \frac{1}{T} \int_0^T |x(k)|^2 dt \tag{3}
$$
where $P$ represents the energy from the signal, $T$ represents the length of the signal lapse and $x(k)$ represents the frequency-domain signal. The spectrum is normalized by the length of the signal.

The PSD is a real function with respect to frequency $k$, so it has the same length of FFT result $x(k)$ if you keepace with Eq. (3).
There are many methods to estimate PSD, but if you want to recreate the results in the paper, you should use exactly the same equation.