Graphics card compatibility: Graphics cards up to 290mm in length with the top HDD cage installed - With the top cage removed, graphics cards up to 430mm in length may be installed
http://www.fractal-design.com/home/product/cases/arc-series/arc-midi-r2
A reference (founders edition) GTX 1080 is 267mm iirc. Some after market ones can be 300mm or more. You'd want to avoid the FE ideally to get something with better cooling. Some after market ones may be short enough to fit, or if a particular model takes your fancy that is too long you can always remove the top HDD cage, assuming it isn't being used or you can fit the drives elsewhere in your case
A reference (founders edition) GTX 1080 is 267mm iirc. Some after market ones can be 300mm or more. You'd want to avoid the FE ideally to get something with better cooling. Some after market ones may be short enough to fit, or if a particular model takes your fancy that is too long you can always remove the top HDD cage, assuming it isn't being used or you can fit the drives elsewhere in your case
11.73 " x 5.28 " x 1.57 " Inch
29.8 x 13.4 x4 Centimeter
https://www.asus.com/uk/Graphics-Cards/ROG-STRIX-GTX1080-O8G-GAMING/specifications/