You can edit the Steam UI's style files to change font sizes. The files
gameoverlay.styles control the look-and-feel of the Steam UI and the overlay UI, respectively. You can locate these by navigating to your Steam folder (usually under
Program Files) and navigating into the
By finding the lines that contain
font-size=, you can change the font size of any UI element.
Specifically, to change the chat font size, find the code blocks that look like this:
And change the
font-size to 14 or whatever is comfortable. There are multiple UI types used in Steam Chat, so you will want to inspect and maybe change the
font-size in multiple code blocks. The names of the code blocks that are relevant are:
You'll have to restart Steam to see the changes.
Be aware that changing these lines is changing the core Steam files, so they'll probably be overwritten when Steam updates. To make these changes stick (and be easier to remove), you might want to look into how to make a Steam skin, and keep your changes in that. (The downside of creating a Steam skin is then you face maintaining it if an update breaks it.)