280 AIO is recommended because it offers similar performance to a 360mm AIO, where 280 AIOs tend to be cheaper than 360 AIOs.
The biggest AIO tested to fit is the ARCTIC Liquid Freezer II 280, but it is a very tight fit. Other more common 280 AIOs from NZXT, Corsair and Deepcool will also fit.
Fans in exhaust or negative pressure setup should yield the best cooling results. This is true for most sandwich layout cases.
Arctic Liquid Freezer II 280 AIO will fit