Don’t know what the max resolution is, but I know for a fact that it can run in 4k, as well as span multimonitor setups. Regardless, if you’re having problems getting it to run in 8k, I would try changing between fixed window/full screen modes to see if that makes any difference, as well as try to manually set the resolution in the config files.
According to my setting available in the game client that I can select it is 8k. But even at 4k things are to small already so you have to scale the UI.