With the game not running, "./username/Saved Games/DCS.openbeta/Config/options.lua" file,
You can change it manually by changing the ["width"] and ["height"] values under ["graphics"] with something like notepad.
Sets the display resolution lower than the display and he starts DCS, it will start at the specified resolution (window size).
For example,
["width"] = 1280,
["height"] = 720,
You can also set ["fullScreen"] = false if you prefer.
When you start it, just change the resolution to your liking from the settings as usual.
By the way, Rename or delete the configuration file, it should start with the initial setting (I think it was full screen)