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Thank you very much Trenton!
Your workaround helps a lot to scale up the font size of plaintext e-mails.

@J. Winters you can even enter a value below 1 to scale down the font size (example: 0.5)

permalink of the workaround:
https://addons.thunderbird.net/en-US/thunderbird/addon/theme-font-size-changer-fixed/reviews/1158832/