Thunderbird doesn't offer the option to let you set absolute font sizes. The reason for this is that email is an electronic communications medium, where your message will be seen, by
and large, on a display screen. You as sender don't know anything about the device your correspondent will use. He may read your message on a smartphone's tiny screen, and later elect to show it on a large display screen during a meeting. In either case, your
instruction to show it in Times Roman at 10pt is meaningless; the message text will be scaled up or down as appropriate to suit the display device (can a smartphone actually display Times Roman?) and the needs of your correspondent. Someone with failing eyesight
may want it in large text and possibly in a specific choice of colours and typeface.
You can specify larger and smaller typefaces to help with emphasis. You can of course use
the decorations offered by the toolbars and menus, but do bear in mind that your correspondent can elect to ignore all 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 Linux computer where I'm writing this web page article, "Helvetica" is the default sans-serif font. If you don't have that font, then your browser will use Arial. If for some reason you don't have this very commonplace font either, then your browser is
invited to use whatever is your system's default sans-serif font. My point is that an email message doesn't contain any formal description of the type style to be used, just a request to use whatever is available. Using an obscure font selection that you have
had to download is pointless, because it is most unlikely that your correspondent will also have it. So their computer will substitute what is considered to be the best match. Fonts are given broad classifications such as serif, sans-serif, cursive, fantasy
or monotype, but some of the "best matches" are somewhat arbitrary.
The default font used when you compose HTML emails is set here:
The fonts used to display messages on your screen are set here:
…and 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.
The size setting
is in pixels, not points. Pixels are generally smaller than points, so you need more of them; try half as much again.
10 pt ≈ 15 px
12 pt ≈ 18 px
14 pt ≈ 21 px
If you don't want to have to accept other people's choice of fonts, clear the box
"Allow messages to use other fonts".
You need to repeat these settings for each character set you use. For English speakers, you need to look at Western, Other Languages,
Unicode and maybe User Defined, plus any that you may use with correspondents who use particular encodings. I've highlighted this with the red outline.