For a 20 MHz channel, with 1200 usable subcarriers(18 MHz occupied), what should be the size of the FFT on the transmitter before IFFT is applied?
We perform M point FFT on each Q block, then map the FFT results to intended positions of 1200 available subcarriers (subcarrier mapping), which are finally the input of a 2048-iFFT operation. See the figure below.



Note that this is just one of possible implementations of SC-FDMA which is called DFT spread OFDM. The another is time domain approach. Check this book (Chapter 14) for more details.