This is easy, but often done incorrectly. The setting is at tools>preferences>fonts&colors. On that screen, set your desired font and size. Then click the Advanced tab. Set all options as you prefer. Notice the top of that pane's dropdown menu. It probably says Latin or Otherwritingsystems. Click Ok. Click the Advanced tab again. Now, click the dropdown menu to the other option (that is, if your first setting was for Latin, change now to Otherwritingsystems). Set all options EXACTLY as you had for the other. When done, your pane for Latin will be identical to that for Otherwritingsystems. See attached images. They represent my personal settings.

NOTE: This font choice and size is for your viewing only. The recipient of your message will receive plain text and will be formatted according to settings on their PC.

Wow, thank you! I had different settings for Latin and other writing systems. When I harmonised them to be the same, the font size when composing non-HTML messages then reflected those settings - which it hadn't before.

You're welcome. It's a widely not-understood aspect of Thunderbird.

Having the same problem, I have left clicked on TOOLS, there are only 2 items on the drop down menu. 1. Mail & Newsgroups Ctl + 1 2. Address Book

Right click and there are two more options 1. Composition Toolbar 2. Customize ...

The customize toolbar has no options to change text or text size. I am using Thunderbird Mail on Ubuntu 20.04 Thank you.

Rodney Jackson.

The menu for Linux is different for some reason (of which I do not know). I think you should click Edit from main tool bar and will see Preferences as an option.

Thank you David, that did the trick. On preferences there is a tick box that gives the ability to male text Small, Normal, Large, Very large. On this reply, I have set it from normal to Large and now even with my poor eyesight I am able to read it without squinting.

Rodney Jackson.

Thanks for feedback. I'm glad it worked for you.