TWO ANNOUNCEMENTS

[Molarpistols]

Twitch, but it's the actual sim engine that won't go full screen on his desktop

It's being streamed through a streaming application to Twitch, yes. Which application is being used to do the streaming?

If it's OBS (Open Broadcasting Software) it's fairly simple to get it at least close to full screen. Create a new "Scene" and under "Sources", right click in the box, click "Add" then navigate to "Window Capture". At the top, a drop down menu appears, and find the simulation window under that. At this point it helps to click "Preview Stream" and then the "Edit Scene" button will allow you to click/drag the size of the window being shown to at least closer to full-screen if not actual full-screen.