Pop XL Silent:
Without a front fan blocking the way of the GPU in Pop XL Silent, you'll have a 455 mm maximum GPU length.
If you plan on installing a radiator, you would need to reduce the maximum length with the thickness of the radiator.
Let's say you remove the front fan, giving you 455 mm maximum GPU length, and then want to install a Celsius+ S24 in the front. The S24 radiator is 30 mm thick, and the fans included with it are 25 mm thick. Adding the radiator and fan thickness together gives you 55 mm. You would then have to reduce these 55 mm from the maximum GPU length, which means that your new max length would be (455 - 55) 400 mm.
Pop XL Air:
If you plan on installing a radiator, you would need to reduce the maximum length with the thickness of the radiator.
Since the Pop XL Air allows you to have the fans on the opposite side of the front fan bracket, all you would need to do is reduce the radiator thickness from the GPU length (unless you plan on doing push-pull). Let's say you use a Celsius+ S24 in the front with a thickness of 30 mm. You then reduce that from the 455 mm maximum GPU length, leaving you with a new max length of (455 - 30) 425 mm.