The maximum GPU length is 368 mm. The maximum GPU width, including cables, is 176 mm.
Note that the maximum GPU length is reduced by any front radiator you might install. If you install a 30 mm thick radiator in the front, the maximum GPU length is (368 - 30), 338 mm.
Pre-installed in the Define One, there is a GPU support bracket on the motherboard plate. This can be adjusted sideways, upwards, downwards, and in/out by adjusting the screws on the backside of the motherboard plate, and the thumbscrew under the support bracket, as seen in instructions below:
Be sure to avoid overtightening it upwards so it puts intense pressure on the GPU, and avoid getting the rubber support of it into the GPU fans by having the PC shut down while adjusting.