You can try manually setting the resolution, but currently the main menu does not scale properly at very wide resolutions.
You can edit the graphicSettings.lsx file (in the '..\Documents\Larian Studios\Divinity Original Sin' folder) in Wordpad, or other text editor, to manually change 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>
