Hey, what's the problem exactly?
Mir's an old game and Chronicles client itself can only go to 1024x768.
It can display/be stretched at any resolution however without issue, you may get some black-bars at the side. This can be fixed by changing some settings in your graphics card settings.
My personal preference is to run Mir in Windows Mode (ALT+ENTER when mirs running) in 1920x1080 resolution. It's large enough to see anything and allows you a bit more freedom elsewhere.