If you read and understand the sticky, I think you already know enough about radiator size needed for your computer. To cool down that i5 3570k and possible 2 GTX 680 along with the motherboard VRM, you would need at minimum 420mm in radiator size and a strong pump (something the like of DDC pump or D5). Since your case support up to 2 large radiator, why not maximize it's potential space and get 2 radiators (a 420 mm radiator on top of the case and a 240 mm on the bottom) ? Doing this will also get you closer to ambient temperature as well since more rad space = better temp.
Your case has a 20 mm hole spacing on top for 420 mm radiator and 15mm hole spacing on top for 360mm radiator, so when you choose a radiator, make sure they have appropriate fan spacing or else you would have to drill holes to make it fit.
HW labs 420 mm radiators all have 20 mm spacing hole which are compatible with this case.
All other 360mm radiators (XSPC, Aquacomputer, Phobya, EK, Switftec, Coolgate, Alphacool etc) all have 15mm spacing hole which are compatible with this case.
When choosing radiator, you need to keep in mind the fin number density of the radiators and select the appropriate fans (higher fin density radiators benefit more using higher RPM/higher static pressure fans). Also keep in mind the noise of those higher RPM fans as noise tend to increase with higher RPM. All these info are in the sticky so if you read it and understand it you should have understand all of what I'm saying in here.
If you have any further question selecting the components, you can post here and we can help you, just don't expect people to hold your hands and explain the entire watercooling concepts (radiator size, thickness, delta, pump, heat dissipation in Watts, fans RPM/static pressure, loop orders, coolant, waterblocks etc) since it will take pages and pages to write them all.