window mode lets you stretch the window across multiple monitors so it should let you stretch it to whatever resolution you want on one monitor.
I am not sure how wide it will go, but there is a breaking point where everything looks stretched out. Also you do not get more game display width size, it is the height that either increases/decreases until it reaches a specific height then everything stretches out. Of course, you can use /viewpoint command so that the graphics part is not stretched so far, and put the UI windows in the empty area.
EQ seems to support up to 8k Resolution, higher then that seems to crash the game (due to too many resources used?), so I'm sure it can fit an ultra wide like that