Speaker sounds with bigger or smaller drivers?

There is a theory that was forwarded by some French audiophiles a couple of decades ago that the surface loudness of a speaker should match the surface loudness of the radiating surface of the instrument that it is trying to reproduce. Hence, a larger driver will be more suitable for reproducing an instrument with a large sound board, like a piano, and a smaller driver is more optimal for, let's say, a flute. YMMV.

Not if you value accuracy over euphonics. The old big woofers just made a lot more noise with flabby control, and uncontrolled self created harmonics.
Modern speakers have far more control, and the pistons do not break up into harmonics easily. So they 'sound' less full. But are in fact more accurate.
If you want the full round luscious bass of old, then by all means buy old speakers with 15" woofers. And power them with a low damping factor amp, to get the most from that floppy boom factory.
Actually plenty of folks enjoy that sound better than accurate 'thin' sound modern speakers produce.
Buy what makes you happy, not what somebody told you to want.

In general, the midrange usually sounds best in the 5" to 8" size range when all things are equal. But its way to hard to say any speaker will sound better or worse do to the driver size. Usually larger drivers are needed for high efficiency, and or higher sound pressures. It's all in the total design that counts in the end.