You can fit bigger ons but you have to remove the fans of the gpu (like cards as the gigabyte Aorus and Asus rog strix) gives better cooling to and no turbelence. You have to fiddle with the fans to link them to the gpu though.is 42mm the absolute maximum or is there enough leeway to fit in a couple extra mm? The GPU i'm looking at is 46mm, just shy of the max thickness.
Pressure. You have restriction not only from how close they are to the GPU, but also from the limited space under the case, so you want fans that are better at pushing through restriction.If putting fans under GPU, is it better to use fans optimized for pressure, or air flow?