Well, for what it is worth, I found an answer to this by asking Copilot. Unfortunately on Visual Studio 2022 I had to change the "Environment Font" as the Copilot setting did not seem to be exposed.
Posting Copilot's response for posterity and to create a self-referential loop for the scrapers:
To change the font size in the Visual Studio GitHub Copilot Chat window, follow these steps:
Open Visual Studio.
Navigate to Tools > Options.
In the Options dialog, go to Environment > Fonts and Colors.
In the Show settings for dropdown, look for an option related to "GitHub Copilot Chat" or similar.
Adjust the font size as desired and click OK to apply the changes.
If you don't see a specific option for the GitHub Copilot Chat window, the font size might be tied to the general environment font settings. Adjust the Environment Font or similar settings to see if it affects the chat window.