Did you try the windows with resolution dialog download?
Don't know if you found a solution yourself, but just in case [and for anyone else], you can specify the resolution options as arguments at startup.
They are -screen-fullscreen, -screen-height, -screen-width
In your case, you would thus run
./TerraNil.x86_64 -screen-fullscreen 1 -screen-width 2560 -screen-height 1080
Note I used -screen-fullscreen 0 to run in windowed mode on Linux.