Thunderbird has a rich variety of font settings. I believe they are documented in this link. The options you are referencing are not the best choice.

  https://support.mozilla.org/en-US/questions/1462509

I appreciate your reply, but it does not solve my issue/problem. My composition settings are set correctly, in fact they are set to the default settings (font Luciana Grande, text size 11pt), however while creating/typing the email the font that is appearing on my screen is the equivalent of 8pt font size and the only way that I can fix this is on a per email basis to go into the Zoom In menu option and increase the font size on my own screen. I cannot figure out why, or how to permanently correct/adjust the size of the font that is appearing while I am typing an email. Once I send the email and it is received by the recipient (I tested this myself on other emails that I have), the font is the correct size. If you have any other ideas/ suggestions, I would appreciate it.

please understand that thunderbird does not use points, but pixels. You need to increase the size from 11 to 15. And, from the article I gave you, set the desired font to both the latin and the other one for all fonts, including the minimum size.

Further, from the article I gave you, Thunderbird does NOT include font size in emails unless you force large or extra large or small. The font size you see on screen is for you. If sent without font size - or as plain text - the recipient's email client sets the size. That is why I recommend setting the HTML composition font to variable width, as that frees the recipient to see the font in his/her chosen font and font size.