Given the native default is to make all text the same size. I guess you have made a rod for your own back by going to the effort to changfe the font size everywhere else and now can not do the last bit.

The correct way to go about scaling is to use the layout.css.devPixelsPerPx not try and over ride font sizes.

The usual way to set the display fonts and sizes for received and composed mail is to set them in Preferences/General/Language & Appearance, Advanced, with Fonts for:Latin and to repeat with Fonts for :Other Writing Systems. Uncheck 'Allow messages to use other fonts'.

Even though I changed the fonts (I only changed the style to Roboto) for folder lists and menus, using the layout.css.devPixelsPerPx setting to scale fonts in Compose/Reply message window does not seem to make much difference. I tried settings as high as 1.50 - it scales my folder list and menu fonts to gigantic size but the text that I am composing still requires a couple of Ctrl+ zooms before it is at a conveniently readable size. I find this really strange because I am not really asking for a very big zoom. I can use Libre Office at Roboto 11 point setting just fine without any zoom (at 100%) and this is what I am using for Thunderbird folder list/menu settings also. I just do not know why the compose message window starts with a very small zoom setting.

your correspondents are probably setting a font size. as SFHowes said, you will need to override their font settings.

Given the native default is to make all text the same size. I guess you have made a rod for your own back by going to the effort to changfe the font size everywhere else and now can not do the last bit. The correct way to go about scaling is to use the layout.css.devPixelsPerPx not try and over ride font sizes.

I would like to change my vote to Matt's original response as I was able to finally indeed fix my problem and get to the right layout! For some reason, it did require a restart (though I only realized that as my Ubuntu deployment required a restart to apply some pending OS updates). Here is what I did.

1. I found all the font size settings that I had changed (filtering config options to only the ones that were changed by me and then further filtering on font) and reverted them back to default. I kept my font selection to Roboto & Roboto Mono. 2. I set the layout.css.devPixelsPerPx setting to 1.20. It now showed my Compose/Reply/Forward message in perfectly readable size without requiring me to use Ctrl+ zoom. 3. I however was resigned to having very enlarged Folder/Message List and Tool Bar font as a result. But a recent OS restart (I presume simply doing a restart on Thunderbird would have done the same thing) seemed to have fixed that and now my Folder/Message Lists and Tool Bar font are back to normal size as before and I still have large readable size for any newly composed text in my Compose/Reply/Forward message window!!