The rendering of the email on the other machines is not under your control. UNLESS you do something like send them PDF (documents.)
Maybe someone can rewrite this to explain that it's not a bug, not broken or why this is so but it's all about you controlling the appearance and sizes on your machine and nothing is wrong about that.
The simplest solution is to send it "plain text." That way it's stripped of font control and what they want, they get.
I have lived with this mysterious, serious problem for years:
The font size and font type which appear on the screens of receivers of my emails are both different from the "normal" ones which I had used (that is, the way they had appeared normally on my screen) while typing the emails.
Only of course a very few of my email correspondents bring this problem to my attention when they find the font size too large. On receiving my emails, they can certainly adjust the font size on their screens as desired, but that's not at all a good "solution" to this problem.
I use Thunderbird for email.
What can I do to remove the above serious, highly annoying problem?