The bigger the fan the better, IMO. I really love those cheap square box fans... It's hard to beat that deep, low hum that small speakers and small fans just can't replicate.
There are 20" square box fans that have a built in slot for a 20x20x1 filter. It's advertised as having a slightly more powerful motor. It is a good noise machine while also getting the added benefit of filtered air instead of just noise
Now that I think of it, back when I was a kid in the 90s, there was a circa-70s oscillating fan that my family kept in the living room during the summer, and the noise it made was quite pleasant. The oscillation added a gentle rhythmic quality that made it even nicer.
I’d never put two and two together until now, but that fan had to have been a major factor in some of the unexpectedly great naps in that room.