The graphics settings seem to be stored in this file:

C:\Users\<your name>\Documents\Larian Studios\Baldur's Gate 3\graphicSettings.lsx

You could close the game, open this file with notepad and change these values to 1280x800 or some such and then restart.

Code
                <node id="ConfigEntry">
                    <attribute id="MapKey" type="FixedString" value="ScreenHeight"/>
                    <attribute id="Type" type="int32" value="0"/>
                    <attribute id="Value" type="int32" value="800"/>
                </node>
                <node id="ConfigEntry">
                    <attribute id="MapKey" type="FixedString" value="ScreenWidth"/>
                    <attribute id="Type" type="int32" value="0"/>
                    <attribute id="Value" type="int32" value="1280"/>
                </node>