Realize that all these coolers are just air coolers, even custom water loops eventually use air blowing across a water filled heat sink to dump air into the room. So all we're really doing is changing the location and size of the radiative surface. CPU coolers are limited by the location of the CPU and the amount of weight the motherboard can realistically handle. AIO's use water as a transport to move the heat from the CPU to a much larger heatsink that is supposed by the case. Custom loop lets you expand the radiative surface to as big as your case allows while also cooling other things like GPUs.
Technically a radiator can be any size, I've seen motorcycle radiators used to cool PC's before, back before everyone decided the 120mm fan was going to be the standard unit with 140mm as the alternative. Radiators need fans and both need to be bolted to the case, so it just makes sense to make them the same size for universal compatibility. 240mm seems to be the most common size, and it makes sense because CPU coolers already come in 120mm sizes, a 120 or 140mm AIO is kind of a waste. 360 is overkill for most every CPU out there, so really not that useful. And no, you shouldn't mix custom loop and AIO. Building a custom loop is something you don't do in half measures, its amazing but fundamentally changes how you approach build and maintenance.