I suggest that an unusually physically large FSL if left untuned (i.e. broadband) would produce sufficient signal output for serious MW DX purposes. But this is academic because such a physically large FSL would be impracticable to transport, operate, and set up at most listening sites.
In a resonant tuned circuit, a high Q factor (L/C ratio) is achieved by low resistance in the main tank coil, hence the advantage of using low as practicable resistance Litz wire. But as the voltage output increases, the RF output signal bandwidth decreases. This potentially can become an issue especially towards the low frequency end of the MW band. Even with my 40 inch tuned square PVC loop, peaking the tuning cap to say 558 KHz results in relatively muffled audio. Assuming the wire overall resistance was lowered, the signal output on 558 MHz would be increased, but the audio would be more muffled.
So like everything else in life, we have to deal with the law of diminishing returns. High voltage output comes with a drawback (decreasing signal bandwidth and associated subjective audio fidelity). A compromise needs to be reached.