To cool the CPU and GPU you are probably going to need two radiators to achieve better than the stock GPU cooler and a separate CPU cooler.
Your CPU, under normal conditions, is pretty efficient, but if you are overclocking or running all cores under a full load that is easily another 200W. 550W nominal dissipation under max conditions.
3090 is no joke, and the Nvidia design is still a huge piece of metal. You don't see any 120mm radiator 3080/3080Ti/3090 for a reason, these things use a lot of power. Base TDP of 350W, with spikes commonly measured in the 500W range.
At reasonable ran speed around 2000 RPM (which is quite audible) EK gives a single standard thickness 360mm (with their quite decent EK Vardar fans) a dissipation capacity just shy of 500W. Mind that is with a more robust pump and under ideal conditions, and when there is a 10C delta, which is somewhat extreme. Probably safer to call it 400W at a more reasonable delta and fan speed.
360mm radiators have a little more surface area than 280mm radiators, so that would be a little worse, probably like 350W, which is enough for the GPU only.
Now, under most circumstances, this is probably fine, chances are you don't have much that can do 100% CPU and GPU load, unless you are running like a RTS at 4K or something. Still, overbuild and have no regrets, or have a loud computer to keep the temperature ranges in check.
So for you I would say go maximum and include also a 240mm radiator in the top.
I use an "AIO" as the core of my water cooling loop, but it is basically an off the shelf DDC pump, and it doesn't sit on the CPU, but in the radiator. I use two 280mm, with one being push pull, and I have my rear fan as intake to put cool air directly into the top mounted exhaust radiator (and give the ram and motherboard VRMs a little air)