Preferences/General/Language & Appearance, Advanced, set the fonts and sizes with Fonts for: Latin, repeat with Fonts for: Other Writing Systems, uncheck 'Allow messages to use other fonts'. This will set the display fonts, but leave the composition faults at their defaults in Preferences/Composition.

http://kb.mozillazine.org/Font_settings_in_Thunderbird

After I go to Preferences / General / language & appearance & Set the default font at Times New Roman size 20, I can’t find any “ok” or “save” button to save the changes.

And then when I create a new email, no changes I attempted to make are noticable.

What am I doing wrong?

Please reply to [removed email from public]

Thank you

You should see a window like the attached picture.

It doesn't work. There is no save but when I restart it shows the new font setting 20!!! I unchecked so that it would use my fonts but it doesn't unfortunately the internet shows all sorts of obsolete methods but with preferences it is easy except that it doesn't work

Here it is

Altho I did what u said & I set my default font to Romans Times 20 (which is a fairly big size), it STILL appears kinda small when I send an email

This setting is for the display and doesn't work

I had restarted my computer and the options had changed; there was an option to allow something to use its own fonts. I unchecked that and restarted. That option was no longer available and it still didn't chance fonts. I had transferred my profile from my windows 7 computer to my windows 10 computer and the settings are the same but the actual font on my new computer is much smaller and difficult to read. Changing the font on my windows 7 computer doesn't work either.

I will re-start my pc & see what happens

I was right. Restarting my pc didn't do anything. (The font is still small even after adjusting it in preferances) In fact, the msg I sent to myself went to the trash folder instead of the inbox. Why is that? How do I fix? THanks

I doubt that going into the trash folder has anything to do with Thunderbird. That's your provider. Evidently changing the fonts in preferences has never worked. It used to be much more complicated and happily they decided to put it in preferences but they apparently never checked it out

It works fine in webmail, but not Thunderbird & I prefer Thunderbird

I solved it; It's not a thunderbird issue. To change your display in Windows 10, select Start Windows logo Start button > Settings Gear-shaped Settings icon > Ease of Access > Display. To make only the text on your screen larger, adjust the slider under Make text bigger. To make everything larger, including images and apps, choose an option from the drop-down menu under Make everything bigger. I set it to 300 and it is similar to what I had in windows 7

Now the thunderbird setting work for messages. I set the font size to 18 and it is fine