The 'problem' is that Thunderbird has more options than other email clients and has the distinct feature of allowing the sender to control preferred font without requiring recipient to accept it.

Okay, this may sound strange, but here is the real way to set fonts on Thunderbird. And I attached examples of my own setting of Calibri 14 as my default. Here goes:

- in Tools>settings>general, set your default font. 
- then click the advanced button. Notice the drop-down menu at top. It probably says 'latin' or 'other writing systems' - this is IMPORTANT: 
- select 'latin from that menu, set your desired font and font size settings, click OK, 
- select 'otherwritingsystems' from the drop-down menu and set the IDENTICAL settings to what you did for Latin, and then click OK. In attached screenshots you will see my latin and otherwritingsystem match.  Notice I also set monospace font.
- and, if you have account set to compose HTML, then set that in tools>settings>composition to variable-width and medium font. (You would only change the HTML font setting if your intent is for recipient to receive your preferred font - not my personal recommendation.)
- Set Sending Format to 'automatic' 

With these settings, your compose window and your sent folders will match. And if you don't write HTML (e.g., bold, italic, color), none will be sent . If you do these settings once, your font will always be consistent. This does not protect you from someone sending you a message in some god-awful font with 30 pt size, but this does provide consistency.

@david, thank you. Changed it both on Latin and otherwritingsystems. Hopefully this helps.

Font sizes were indeed 14 or even 17 on Monospace on both sections [where my default is 11] 😮

In addition:

The 'Proportional' size you choose eg: 14 will be what Thunderbird compose/write message deems as 'medium'.

The 'Minimum Size' controls the size of the smallest allowed font size which gets displayed in the Message Pane and also in the composing Write window. It is important to set the 'minimum size' to less than or same as proportional - never greater otherwise it will overrule all your settings.

In my case, I set 'Proportional' and 'Monospaced' as 16, but set 'Minimum size' as 12. So if someone send me an email with stupidly small font size of 9 then it will get auto displayed as a 12.

It also means when I compose an email, I can choose to make a font size which is a bit smaller, but it will not allow a font size less than 12.

As david mentions, anything in html css set as medium or not specified will use whatever is set up as 'Proportional' size. Anything smaller than that then falls under the 'minimised size' setting, but anything larger - well if sender actually set a large header then nothing you can do about it.

Always make sure that whatever is set up for 'Latin' which is 'western encoding' is also set up identically for 'Other Writing Systems' which is 'unicode encoding'. It also follows to do the same for any other 'Font for' option which you may use.