How to determine the FFT size from Subcarrier spacing and channel bandwidth?

Hello pros,
The maximum FFT size in 5G NR is 4096, but how do we calculate the size from the channel bandwidth and the required subcarrier spacing??

I don’t think that the FFT size is specified whatsoever.

FR1 cell bandwidth max 100Mhz.
For example, 100Mhz scs 30khz, 273RB.
No of sub carriers = 273*12 = 3276
FFT 2048 is not enough for sampling 3276 subcarriers, must be at least fft 4096.

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But for 60mhz, the number of subcarriers is 1944, that means 2048 fft size is suitable but we use 4096 fft size, why

Nothing prevents to use higher FFT.