Did you try 1280x720? That may avoid the blurriness of 1920x1080.
Did you try 2560x1440 in fake fullscreen mode?
Why are you using a configuration tool in Steam? Does 2560x1440 not show up in the options in-game? Is your desktop set to 2560x1440?
Did you check your video driver settings, to see if you had any stretching or scaling options set?
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>