I am trying to detect unknown RF tones around -140 dBm and my scan BW is 5 MHz, going through the Noise power calculations the signal is below the thermal noise based on the scan BW. I read that using large FFTs can help to pull signals out of the noise floor. My question is how large FFTs can accomplish this?

The receiver used is an RFIC (LNA, Filter and ADC), and I was thinking FFTs of the order of 2¹⁶ or higher running on a hardware accelerator.

The unknown RF signals are ERP emissions from electronic devices which have RF circuitry.