Try lowering your desktop resolution or disable any secondary monitors, etc, to see if that will get D:OS to lower the game resolution.
Try deleting the graphicSettings.lsx file, in the '~/Documents/Larian Studios/Divinity Original Sin' folder, to get the game to recreate it when it is next started.
Alternately, you can edit the file in a text editor to manually set the resolution:
Search for the section:
</node>
<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>
Search for 'fake' near the start of the file to set the display mode (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>
<node id="ConfigEntry">
<attribute id="MapKey" value="FrameCapFPS" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="60" type="4" />
</node>
<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>