Let me try to give some more detailed instructions:
First, go to “Help” --> “Troubleshooting Information”, find the entry “Profile Folder”, click the button “Open Folder” next to “Profile Folder”. It will open the profile folder.
Now you should see some folders like “crashes”, “datareporting”, “minidumps”, “safebrowsing” etc.
(By the way, it is a good idea to make a safety copy of the complete profile folder.)
Create a new folder named “chrome”, so that it appears alongside with the folders named above, i.e. it appears in the same folder as the folders named above.
Inside the chrome folder, create (e.g. with notepad.exe) the file “userChrome.css” with the content:
font-size: 12pt !important;
(Please make sure that the file is named “userChrome.css” and not “userChrome.css.txt”. Maybe you should google "how to make windows show file extensions.)
Next, in Thunderbird you need to open “Preferences” --> “General” --> “Config Editor…”, accept the warning dialog.
Search for the entry “toolkit.legacyUserProfileCustomizations.stylesheets” and double click it to set its value to true.
Finally, restart your Thunderbird.