STFT Window Size is 2048, Why is the output 1025?
The continuous Fourier transform possesses symmetries  when computed on real signals (Hermitian symmetry). The discrete version, an FFT (of even length) possesses a slighty twisted symmetry. 

The DC coefficient ($F(0)$) is real, as well as the Nyquist one ($F(N/2)$). In between, you get $\frac{2048-2}{2}=1023$ "complex" coefficients, "duplicated" in positive and negative frequencies. 

So for real signal, each STFT frame can be represented by $1023+2$ frequency bins, the remaining   1023 being recovered by Hermitian symmetry.

You can get complementary information at FFT of Pure Real Sequences:


  As a result, we can see that if N is even, both $F(0)$ and $F(N/2)$ must
  be real. Given these two values and the complex values $F(1)...F(N/2-1)$,
  (I.E. N numbers in total) the sequence is completely characterised.