Assuming you're on PC, you could always use the Windows magnifier in something like lens view to magnify only what's under your mouse. This would probably work even on your second screen. You'd just need to enable and disable it at will. Windows and plus to start it, and windows escape to close it.

Supernova Magnification has $1 supports $1 differentzoom and other settings as well as in Zoomtext has Freeze Area Mode and $1 both are having demo version to try out. Windows Magnification doesn't have such advanced features but it's lighter to resources and it's free. Both Zoomtext and Supernova licenses are as pricey as new gaming laptop.

Ok, so I can't send you an image. At all. Wanted to show you how I'm using the docked setup for when I work in Blender to do my 3D animation.

I have docked mode pretty much taking up the entire second screen, and even though it still follows my mouse, my other monitor is basically unaffectedz so I can have a normal workspace.
If I had thought of this sooner, I'd have *way* less troubles in online games, lol!!
Then again, during those years, magnifier did not want to work well with any other program.

Many games do have the ability to set font size, window opacity, and UI resize to increase the size of the chat area.

Otherwise, a thought might be to use an app like Discord, Google Meet, OBS, etc., to cast your game application, and put that app in the second window with the focus on the chat portion of the game.

Really though, magnification and dual monitors just don't really seem to want to mesh well together.

Windows Magnifier will do this, works very well (on Windows 10, anyways), and comes with the operating system for free. You can use it three ways: Full screen (everything is bigger, and follows the mouse cursor), Lens (you have a box of magnification that follows the mouse cursor), or Docked (a stationary window of magnification, contents follow the mouse cursor).

So for example, I worked with a woman yesterday and she had a triple monitor setup. We set one monitor as the "magnifier" and docked the magnifier app on that one. (Set magnifier to automatically start and set her needed power of 2x) and made the dock the entire 3rd monitor. She now has two monitors where things are normal sized and one that show anything she touches with the mouse on monitor 1 or 2 at double the size.

SuperNova and ZoomText will both do this also, but both of those are going to cost you; try the built-in magnifer first.

Some games have the ability to move chat boxes to a different screen or change font size. Check the display options or contact the company with feedback.

So, Windows magnifier Docked mode... Sort of does what you need it to do. Though, positioning it exactly over a chat box might not be super easy. You could always have it follow your mouse, but have the dock on a separate screen.
Am I making sense? Or just being confusing? Lol

Ontopreplica does that