Apple Mail Font size problems solved
There are various discussions about the very small font size in Apple Mail. eg
Apple Emails printing with VERY small font - Apple Community
Apple Mail font size too small for father - Apple Community
No one seemed to know how to solve the problem.
It turns out that Mail font is not too small after all - it is just using a different unit of measurement - 'pixels', whereas anyone used to the Microsoft world and Outlook uses 'points'.
12 points in Outlook equates to 16 pixels in Apple Mail. So if you use font size 12 in Apple it will be too small.
This doesn't just apply to Apple; other programs like Proton use pixels, too.
See https://blog.gimm.io/difference-between-pixel-px-and-point-pt-font-sizes-in-email-signatures/
One wonders why Apple (unlike Microsoft), in the mail settings, couldn't make clear the units used. :-(
(eg Change Fonts & Colours settings in Mail on Mac)
Hope this helps someone else!
iMac 27″ 5K