Font size is an issue that is difficult to explain to windows users because there is non as such. Your email is being composed in HTML and that markup language does not really use font sizes as windows does. While it is possible to make absolute statements to force font sizes into the familiar points, it is counter productive. Microsoft do allow it in their mail clients as they are designed for internal use and a Microsoft proprietary message body. It at time results in some very odd looking emails out in the internet, usually from corporates that impose corporate livery unsuccessfully into email.

You have no control over where your email will be read, or on what sort of device, it might be a low resolution 15" display or a very high resolution 4" display on an iPhone or even someones 65" television. To cover these eventualities HTML has font sizing in 6 values which appear in the Thunderbird user interface as larger and smaller with "normal/ medium" being the starting point. This allows the receiving device to render the font at the rendering engines optimal size for the device.

Likewise font names are not necessarily the best was to communicate as Linux in particular often does not have that specialist font installed so HTML introduces the scope of font families. So unless you take action the font will be describes as a variable width sans serif font and it will be left to the receiving device to determine what that font is actually called.

In options > composition there is a setting in the general tab to set a preferred font and font size. The default size being "Medium" you also get options for text colour, text background colour and the ability to set the default style to body text or Paragraph. (Paragraph having a double space when you hit enter)

Clking Smaller or Larger font size has no effect in my Thunderbird emails.