In your 3rd picture, click the Advanced button to set the display sizes for your desired fonts and see if that improves the appearance on your system. How your messages are displayed at the recipients' end depends on how they have set up their systems.

http://kb.mozillazine.org/Font_settings_in_Thunderbird#Message_display

http://kb.mozillazine.org/Font_settings_in_Thunderbird#Message_composition

Thanks for the reply! No luck, unfortunately. Attached is the screenshot of the Preferences > Display > Fonts & Colors > Advanced... window.

Am not worried about the remote system, which seems to render my mails fine to the readers; am focused on why 11pt Calibri renders as what feels like 6pt on my system.

The size in the fonts settings is in pixels, not points, so you should set it to something like 20. See the section near the end of this article:

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

Well, that makes sense to me now. I need 15 pixels for 11 points on a Mac. Probably different on Windows. Thank you for that link!

In my case, I want to use Calibri for emails going to coworkers, as 99% of my mails are read in mail readers which default to Calibri for reading and composing... so this was an interesting journey.

I submit that perhaps the Mozilla UI in 63+ could put an extra sentence in the Preferences > Display > Fonts & Colors > Advanced... window stating display values are in pixels, not points. Or just state the unit type next to the numeric value.