This guide will walk you through the process of changing the UI scale on your Steam Deck with the DeckHD screen installed. By default, the UI appears smaller than normal when the DeckHD screen is installed due to the larger resolution.
In SteamOS, go to Settings -> System -> toggle On Enable Developer Mode
In the Settings menu, scroll all the way down to Developer and toggle On CEF Remote Debugging. Your Steam Deck will commence a restart.
Install and launch Google Chrome. There are two ways you can do this:
Input the following web address in Google Chrome: http://127.0.0.1:8080
A web page with the title Inspectable WebContents will load. Click on any of the following hyperlinks (e.g. Game Root Menu)
On the right hand side you will see a web console UI. Click on the double arrow and select Application
Resize the column to reveal the full window. Go to Local Storage -> https://steamloopback.host -> AllowInternalDisplayScaling, change the Value from “null” to “true” as shown.
Restart your Steam Deck.
In SteamOS, go to Settings -> Display -> toggle off Automatically Scale User Interface, then play with the slider until you’re happy with the display size.