That add-on you mention is for the user interface; it governs the font used for the main window, the controls, the lists of accounts and messages, and so on.

The display of the content of messages is managed elsewhere. And it's a bit more complicated.

First of all, if you send plain text messages, there is no mention of fonts in your sent messages. How the message appears in your correspondent's computer is totally at his/her discretion. However you can set the font and size used to display it to you when you compose it to suit your own needs or comfort.

However you are more likely to be using HTML to compose, since this is the default setting, and most users seem to expect to be able to choose fonts and sizes in email.

The polite way to go about it is to send messages composed in a "medium" or "regular" font size, then it's up to the reader to choose a display size to suit them. So you can set it to large text if you have shortcoming with your eyesight, yet not inflict your own choice on the recipient. Likewise, if everyone sent to you using "regular" or "medium" sized font, the size at which you see it it would be your choice, and not imposed on you by the sender. (However a great many users come here to complain that Thunderbird doesn't let them set a specific font size. And quite right too, IMHO!)

What this all comes down to is that Thunderbird has two sets of adjustments for the text of messages. One for display, and another that influences the formatting of the messages you send.

http://xenos-email-notes.simplesite.com/417754775

I have read your detailed reply, have checked my settings, but still when I am typing a message it is "ant" sized. When I check Sent it looks normal sized, but it is my display that is so tiny. (In Options, I have the type size set at 14 pt. Verdana and 15 pt. Verdana on Theme Font Size Changer.)

I believe that I am missing a setting somewhere as I did not have this problem with Windows 7, but can't remember all my prior settings.

Another thought: I have reset the type style to Helvetica, Arial, then went into Format>size>larger and this worked quite a bit, but it won't SAVE. Is anyway to set it to Larger? I could work with this size, but hate having to change it everytime I write a message. Thanks.

First those settings are in pixels, not points. If you have a high-resolution screen, you may need rather bigger numbers than you are used to. I thought I'd covered that in my linked article.

Format|Size|Large will send <big><big>BIG CHARACTERS</big></big> to your correspondents, and the real issue is to get them to a size that suits you.