Here are some steps to return the font size back to the original, or really to anything you want:
This should change it back to normal if you did it right. I'm not sure if this will go back to default every time you update, but if it does, you can always come back here and fix it again with these steps.1. Go to this folder: "C:\Program Files (x86)\Steam\resource\styles" and find the file that says "steam.styles".
2. You will be prompted to open it with a program. Choose notepad or notepad++.
3. Use Ctrl+F, search for, "// styles for text used in RichText controls in various places".
4. Where it says "friends_chat_text" all the way to "friends_chat_securitylink", just replace all the instances of "font-size=16" with "font-size=14" (or whatever size you prefer).
5. Save the file and close it, then restart steam.