If you have a closed-loop water cooler then you can place the radiator at a 120MM Fan mount just make sure that the radiator is the same size as your fan mount. Just choose a single radiator if you have a single fan mount, and choose a dual radiator if you have two side-by-side fan mounts
e.g 120MM mount = 120MM Radiator
140MM mount = 140MM Radiator
some radiators won't fit in some cases because they may be too wide or thick.
if you're not sure go to pcpartpicker.com
choose your mobo, case and water cooler, it's easier there because they have a compatibility filter