Best radiator size

Thicker rads ( or thin rads with high FPI) need higher pressure rated fans to cool them.
Here's a great comparison chart of Rads to give you an idea, since the answer depends on pressure ratings of fans, among other things as well.

http://www.xtremerigs.net/2015/02/11/radiator-round-2015/10/

I'm not an expert, but the way I see it, the second loop is best used if the main loop is too big (too high of head pressure) for the pump to maintain a suitable turnover rate. Then you break the large loop into 2, adding a second pump to increase flow rates. They may also be beneficial if your case is sectioned and you can drop a loop in the closed section away from the motherboard chamber. This allows you to siphon heat from the main compartment to a cooler area for exhaust, lessening case temps.
Other than that, I believe it's just a way to show off

If you look at the chart I provided a link to, it shows the flow restriction of each Rad. The higher that restriction, the more workload it adds to the pump (by way of head pressure), possibly requiring the use of a bigger or secondary pump to manage flow. This restriction rises considerably with each block/rad you add. If you use a few low flow rads and blocks, you might find yourself needing more than a single pump can manage.

The goal here is to be able to run your selected pump(s) at a minimum flow of 1 gpm (gallon per minute).
Here's a great post on figuring out how much pump you should need:

http://www.overclock.net/t/1108918/what-can-my-pump-handle-a-guide

Since thickness is not an issue, you might also look into the benefits of push/pull configs on rads, which adds fans to both sides of the rad for better air flow at lower fan speeds (AKA quieter).

Then run the bottom as a super thick push/pull config and run the top rad pushing heat out with a single.
The large thick rad will be handling most of the heat, so make it as efficient as possible with high pressure push/pull fans. This negates the necessity of needing more cooling up top, so a single exhaust fan on a high-quality rad would be fine to cool the cpu.

+1 Wizard.

You implement your cooling solution around what you're cooling, not around what can fit in the case. If you want max OC with max heat generated from your CPU and you're running multiple OC'd GPUs, yes, dual loops can be beneficial so the heat of your CPU doesn't impact your GPUs, and vice versa. However, with pascal GPUs not really being able to generate all that much heat due to locked voltages pretty much across the board even when OC'ing, I personally (just my opinion) don't see the need for multiple loops even in a dual GPU setup. Just put your CPU first in the loop after your pump and get a killer radiator after the CPU, then flow through your GPU(s) and get a decent 2nd radiator after the GPU, then return the liquid to your res.

I cool a custom build I do (niche build, not for gaming) with 4 x 1080ti's and a 2011-3 CPU. Cooling the GPUs is priority in that loop and there's no OC on the CPU. The applications that run will toll the GPUs (no its not a mining machine...) but not the CPU at the same time. I push from the pump through the GPUs first (using an EK semi-parallel bridge), then through a 280mm rad, then the CPU, then another 280mm rad then the res. Works like a charm but the CPU does definitely warm up when the GPUs are under load, just from the warmer liquid flowing over its block. That's fine though as its still in the 50s and the CPU isn't doing anything at the time. IF I had an OC on the CPU as well, or it were running at the same time, I'd either have 2 loops in that build or find a case that could support more, and larger, radiators to provide more cooling. Point is, the cooling design needs to fit what you're actually trying to cool.

You mentioned silence. The best way to achieve this is with radiators that take 140mm fans, not 120s. As far as fans go, I'm a huge fan of the new ML140 pro (and ML120 pro) from Corsair. There are LED options, they look great and their static pressure is awesome. They, like ANY fan, will go up in noise as they need to spin up to higher RPMs but they have a great sound profile in my opinion.

So, what's your CPU? What's your GPU(s)? Will you be OC'ing? What is your actual case model?