If I lower my desktop resolution the game will limit the listed resolutions to that value.
You can try setting the resolution manually. Edit the graphicSettings.lsx file (in the '..\Documents\Larian Studios\Divinity Original Sin' folder) in Wordpad, or other text editor, and search for the section below.
</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>