You have to understand that the reason you can't just find a simple answer is because there isn't one. Don't worry it's not complicated but it's something you need to understand.
The amount of heat a radiator (hence loop) can dissipate ultimately rests on the frontal area and the air flow rate through it. Thickness, FPI (fin density), construction, water flow rate etc.. are factors but ultimately not significant. You will find that serious radiator reviews graph heat dissipation (watts) against fan speed for different delta temperatures.
Delta T is the difference between the temperature of the air going through the radiator and the temperature of the coolant in the loop. The more heat the radiator dissipates compared with the heat being added to the loop by the components the lower the delta T will be. And hence the lower the temperature of the component will run at, but the Delta can never be 0 unless no heat is added. 5°C is overkill, 10°C is considered pretty good, 15°C would be ok, probably the worst you should aim for.
So when picking a radiator you need to balance these factors of fan speed, radiator frontal area and delta T to get the right solution for your needs. Once you know how much heat you need to get rid of, how fast you want the fans to run and what temperature your happy with you can gauge how much radiator area you will need. Or if your limited by radiator area you can see what fan speed would be needed etc...
If you have a 4690k and a 970 then the TDP combined is around 250W, you can expect that to increase by a significant amount if you plan to overclock heavily and the pump will also add heat to the loop, not enough to consider but it's important to note the coolant in most cases is used to cool and lubricate the pump. Although this newer tech is more efficient stock the increase in power consumption when overclock is still rather profound.
Above you can see some heat dissipation analysis of a 360mm radiator and with a delta of 10°C and 250-300W of heat your in a sensible fan speed range. You could always let the delta T rise a little and run the fans a bit slower, there is headroom in that respect. I really don't think just a 240/280mm will give you enough heat dissipation at a reasonable fan speed and temperature. And to concur with that has already been said (I guess the long ass reply was unnecessary but at least you know a little more now) you should probably go for a bigger radiator. Bigger as in 360 not thickness. If you want it to be uber quiet and overclock hard go even bigger or multiple rads. Running a 240 + 120mm rad is a false economy really, 120's are expensive for what they can do plus still need two fittings, two tubing runs etc.
I hope you find the most satisfactory way to fulfil the criteria of your build and good luck with the loop
If you want further help provide more details. I would go for a D5 kit btw and XSPC AX radiators are epic quality.
JR
