I don't see it either, I was able to change it within the graphicSettings.LSX file.

"Documents\Larian Studios\Divinity Original Sin Enhanced Edition"

Open "graphicSettings.LSX" with a text editor, CTRL+F "ScreenHeight".

Code
                    <attribute id="MapKey" value="ScreenHeight" type="22" />
                    <attribute id="Type" value="0" type="5" />
                    <attribute id="Value" value="1080" type="4" />

                    <attribute id="MapKey" value="ScreenWidth" type="22" />
                    <attribute id="Type" value="0" type="5" />
                    <attribute id="Value" value="1920" type="4" />

Change the value.
I.E., Change the "1080" to "720", change "1920" to "1280".