The maximum GPU length in the case is 413 mm with and without a fan installed (fans installed on opposite side of the front fan bracket, causing no length restriction for GPU). If installing a radiator in the front, you need to reduce the 413 mm with the thickness of the radiator, meaning that if you have a radiator that's 30 mm thick, the maximum GPU length would be (413 - 30) 383 mm.
Due to the motherboard tray being offset inwards in the case, the maximum GPU width is different from the standard North XL. The maximum is, with cabling for the GPU, 169 mm.
If installing a Flex 2 for vertical GPU mounting, note that the Flex 2 reduce the maximum GPU length by 15 mm (bracket is slightly offset into the case), giving you a native maximum length of 398 mm.