I have xfce with Ubuntu (i.e. Xubuntu). In my installation the font size of Thunderbird menus, message listings etc. is determined by the xfce desktop 'default font'. (xfce Settings > Appearance > Fonts). That controls everything in Thunderbird apart from the text of messages, which is set within Thunderbird itself, so mine is not exactly the same as yours. However you may want to experiment with themes and font sizes in xfce to see if that solves your problem.

I have spent weeks in the past trying what you suggest, I have xfce setup exactly how I want it but thunderbird always seems to adjust to about 60% of the hight required. If I set default font size to 30 then text is to big but in thunderbird it is to big and half missing. If I set it to 8 then it is to small to read and in thunderbird it is to small and half missing still.

There's a thread here that may help: https://support.mozilla.org/en-US/questions/1087338 You'll need to read the posts towards the end where Toad-Hall gives instructions to create a userChrome.css file within your profile. This is a configuration file which determines the font size to be used within Thunderbird, it does not affect the rest of your desktop settings. If you haven't tried it already it might be worth a shot: once you have the .css file in your profile you can re-edit it again and again to tweak the font size pixel by pixel.

All that seems to do is globally change the font size everywhere in tbird but the menus and message bars still only display the top half of the text.