How to determine the original size of the FFT used in unknown OFDM symbol
If you use an FFT size to generate a PSD estimate far longer than the IFFT used to generate the OFDM signal, then you'll see the "valleys" between the sinc maxima in the spectrum, and then it's easy to just go in there and count subcarriers, and their spacing.

This will, however, require a high degree of frequency stability in your system, and won't work well with OFDM systems where small number of consecutive symbols are bursted out.

You can make that a little more robust and work on longer autocorrelation functions, like here.

You probably want one of the more advanced methods of OFDM parameter estimation, like those implemented in gr-inspector (hint: that's free&amp;open source; don't reinvent the wheel).

The relevant blog post of the author of that software  would be https://grinspector.wordpress.com/2016/07/08/week-7-ofdm-prototype/ . 

You basically want to read


  Bouzegzi, A., Ciblat, P., &amp; Jallon, P. (2010). New algorithms for blind recognition of OFDM based systems. Signal Processing, 90(3), 900–913. doi:10.1016/j.sigpro.2009.09.017


They propose clever (as in: outperforming above roughly sketched autocorrelation-based methods) ways to synchronize and estimate OFDM parameters.