The PC version can use the controller UI, but I'm not sure how that would work for streaming.
There is a global scaling factor in the keyboard UI, but the UI is already designed to maximise screen usage, so you can not make it very much bigger without running into issues with clipping.
Open the config.lsx file (in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>' folder) in Wordpad, or other text editor, and search for the term 'UIScaling', then change the value in value="1" two lines below. At 1.1, for example, part of the load button in the load windows is cut off, and the bottom of the trade window is cut off (though it is still possible to click through the hotbar blindly to accept a trade).
<attribute id="MapKey" value="UIScaling" type="22" />
<attribute id="Type" value="2" type="5" />
<attribute id="Value" value="1" type="6" />