Look in C/users/you/savedgames/DCS/Config/options.lua
edit the options lua in notepad++ and manually input the screensize required in the lua
lines 55 and 67
["graphics"] = {
["DOF"] = 0,
["HDR"] = 0,
["LensEffects"] = 0,
["MSAA"] = 3,
["OculusRift"] = true,
["anisotropy"] = 2,
["aspect"] = 1.7777777777778,
["civTraffic"] = "low",
["clouds"] = 1,
["clutterMaxDistance"] = 1500,
["disableAero"] = false,
["effects"] = 3,
["flatTerrainShadows"] = 0,
["fullScreen"] = true,
["heatBlr"] = 1,
["height"] = 1080,
["lights"] = 2,
["multiMonitorSetup"] = "1camera",
["preloadRadius"] = 29116,
["shadowTree"] = false,
["shadows"] = 0,
["sync"] = true,
["terrainTextures"] = "max",
["textures"] = 2,
["treesVisibility"] = 4749,
["visibRange"] = "Medium",
["water"] = 0,
["width"] = 1920,
},