The default font sizes in Thunderbird are fairly small, especially on todays large high resolution monitors. You may need to change these for more comfortable viewing. Setting the font sizes in Thunderbird for message text is easy.
Use the Menu Bar to open the Edit menu and click on Preferences which is at the bottom. In the Thunderbird Preferences menu click on the Display tab. Towards the bottom of the Thunderbird Preferences menu, shown in Figure 1, click on the Fonts button. This opens the Fonts & Encoding window.
You can set the font sizes for the three proportional fonts and the monospace font. Simply click Size selection drop-down menu for the font you desire to change and select the font size.
To set the minimum font size for any text in the message select the Minimum font size drop down and select the desired font size. When you have finished selecting the font sizes, click on the OK button to complete the selection. This will save the changes and make the change immediately.
Note that all of the proportional font sizes are the same. You cannot choose one size for Serif and another for Sans-serif, for example.
You can also use this menu to change the font face in the same way. Click on the font drop downs and select the font face you want.
Setting the font sizes for the message and folder lists is not so easy and should only be attempted by more advanced users who are comfortable using the command line interface (CLI) and a standard text editor such as vim or kedit. See the document Setting User Interface Fonts for Firefox and Thunderbird for details of setting those font sizes.