If you plan to go the all in one route, try to get a case that will fit a 2 x 120(120x240) or 2 x 140(140 x 280) radiator because they cool better than the single 120mm units do.
If you are going custom you can use more than one radiator to give tons of cooling potential(some case modding may be needed).
As the above user says, every case is a bit different.
You want to also ensure you have room for the rad and fan or fans(it tends to get thick with push-pull fans). Custom loops can also use very thick radiators so this must be taken into consideration.
When in a pinch, you can always mod a case to take parts you want to use