Thunderbird cannot set absolute font and sizes.
Your dictates for font and size are only meaningful for you,
since the message will be displayed for the recipient according to his
media and his own preferences.
The message text will this way be scaled up or down as appropriate to suit
the display device, and fonts will be substituted as required.
Meaningful changes you may do and expect them to carry over to the recipient
are specifying larger and smaller typefaces to help with emphasis.
You may also use the decorations offered by the toolbars and menus,
but do bear in mind that your correspondent can elect to ignore these
settings. It is not safe to rely on text decorations to make your point.
The way that formatting is added to an email message is by use of HTML tags. Here's a sample of the type of HTML code that might be used to request Helvetica font:
<span style="font-family: Helvetica,Arial,sans-serif;">Please see my notes below on setting fonts for multiple character sets.</span>
This means that "Helvetica" is the default sans-serif font. If you don't have
it, then Arial will be used. If you don't have this either, then whatever
is your system's default sans-serif font will be used.
The default font used when you compose HTML emails is set here:
Tools | Options | Composition | General → HTML
The fonts used to display messages on your screen are set here:
Tools | Options | Display | Formatting
If you don't want to have to accept other people's choice of fonts,
press the Advanced... button and clear the check-box
"Allow messages to use other fonts".
Note that these two settings don't necessarily have to agree.
You may well compose in Times Roman and yet view your messages in Arial.