Delete or rename the graphicSettings.lsx file, in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition' folder, to get the game to recreate it at startup, reverting to the initial fullscreen resolution.
It is also possible to edit the file, in Wordpad or other text editor, to manually change the resolution and display mode.
Search for 'fake' near the start of the file to set the display mode. Edit the number in value="0" or value="1" in the second line after FakeFullscreenEnabled and / or Fullscreen; if both FakeFullscreen and Fullscreen are set to zero, it will be in Window mode).
<node id="ConfigEntry">
<attribute id="MapKey" value="FakeFullscreenEnabled" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="0" type="4" />
</node>
(and a couple entries lower)
<node id="ConfigEntry">
<attribute id="MapKey" value="Fullscreen" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="1" type="4" />
</node>
To change the resolution (though it may be easier changing it in-game if changing the display mode get the game to start), search for 'Height', and edit the resolution in the value="xxxx" terms (shown below for 2560x1600 resolution):
<node id="ConfigEntry">
<attribute id="MapKey" value="ScreenHeight" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="1600" type="4" />
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="ScreenWidth" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="2560" type="4" />
</node>