How big of a radiator do I need?

This answer varies based on the heat load of your loop and your specific goals with the loop. It will also vary based on the performance of the fans you intend to use and the airflow restriction (if any) of your case. A good starting place these days is a 240mm radiator per component. 120mm per will frequently work, but not as well as most pursuing custom liquid cooling will want. More radiator space is better, generally. A 360mm+ radiator per major component setup will be great for those hunting near silence or looking for more overclocking headroom. Luckily, many cases are now being designed with dual or even triple 360mm radiator support in mind.