My case can take either 280mm in front and top or 340mm in front... I picked up a corsair h110i for my top and I am running my 1700 on asrock x370 taichi, I am currently at 3900 with less then 58 degrees at 100% core usage @ 1.35 v. I don't see how anything bigger would make any real world difference. I don't even have chassis fans yet, just the h110i. Just my 2 cents from personal experience.
longer is better. When you have a 240 and upgrade it to 360 it's a world of difference. That extra 120mm really does work! Where-as 280mm vs. 240mm is minuscule at best. Unless the fins per inch and you have a "best setup" for fans/radiator, I'd go with 360mm. I mean if you can try 480mm if you won't have regrets!
240/280 are not really much better than air either. My NH-D14 is beating many 240 aio coolers... And there is also D15, that is even better... So yeah...
I would use 360 if I can foe 2 reasons...
1. Overkill in cooling is never a bad thing. I use D14 for my Athlon 760K and @100% it can't reach 50C...
2. Large surface area, lots of fans, low rpm on the fans, quiet operation.