The font sizes you are seeing are dependent on the size defined by the HTML of the e-mail. If you cannot find an option to enlarge the font size in Thunderbird, try using Windows Key + and - in Windows 7 to make use of the magnifier for hard to read e-mails.
The problem of difficult to read font size is the result of monitor resolution changes taking place. While many users are now using high resolution, hi-definition monitors, some are still using older, low resolution monitors. This has resulted in a disparity that requires many publishers to display their content with a smaller font, in order to accomodate words that would appear too large on the screen.
The majority of Internet users are still not using hi-definition, high resolution monitors, but this number is growing, resulting in disproportioned content. It is recommended that you make use of the program's own magnification abilities, and the larger and smaller font sizes under Display Properties. When that is not possible, it is recommended that you use the magnifier option in Windows.
