First and second are related.
A 240mm AIO has a capacity of @ 250w, same as big air like a Noctua NH-D15. A 360mm has a capacity of @ 350w.
But that's capacity and if the cpu cannot reach over 200w outputs, then actual capacity ceiling is next to meaningless.
What matters more in that respect is Efficiency. A beQuiet Darkrock Pro 4 is the same 250w as a NH-D15/S. The D15 gets better temps based solely on higher efficiency, a better match up between fan/s performance and heatsink characteristics. A Noctua NH-U12S is half the size of a D15, but @ 100w or less gets better temps than it's bigger brother, simply due to efficiency.
Most AIO's radiators come from the same few OEMs, so no matter the brand the rad is functionally identical. Which puts efficiency all on the particular fans used, the case airflow characteristics, orientation and the fan curves applied.
In one case a 240mm might get better temps than a 360mm under loads, change cases or placement or orientation and the same rads might get opposite results.
There is no 'best' as such, much of it is opinion. I prefer the Nzxt Kraken on Silent mode, even though Performance mode gets 3-4°C better temps at load, but the fans noise is a deciding factor.
Bigger can be better, but size is not a guarantee of performance.