This is not exactly the same as the minimum font size setting that was previously available, but it can rectify the issue in most irritating cases. (Namely, email from outlook users)
Mail.app has a CSS resource file that it loads in the Webkit view used to render messages. You can find it in the Mail.app bundle here:
/Applications/Mail.app/Contents/Resources/Message.css
You can add a simple CSS rule to improve your reading experience. (You could go crazy affecting styles in here too if you were so inclined...)
The biggest offender for tiny message text I've seen comes from using the size attribute on font tags. Adding this rule resolves that issue without being too opinionated about font sizes otherwise:
font[size="1"],
font[size="2"],
font[size="3"],
[style*="font-size:5px"],
[style*="font-size: 5px"],
[style*="font-size:6px"],
[style*="font-size: 6px"],
[style*="font-size:7px"],
[style*="font-size: 7px"],
[style*="font-size:8px"],
[style*="font-size: 8px"],
[style*="font-size:9px"],
[style*="font-size: 9px"],
[style*="font-size:10px"],
[style*="font-size: 10px"],
[style*="font-size:11px"],
[style*="font-size: 11px"],
[style*="font-size:small"],
[style*="font-size: small"],
[style*="font-size:x-small"],
[style*="font-size: x-small"],
[style*="font-size:xx-small"],
[style*="font-size: xx-small"] {
font-size: inherit !important;
}
You need root privileges to edit this file. Open it in TextWrangler/BBEdit or another text editor that allows you to save with an admin password, or edit in the shell with sudo:
sudo echo -e "\nfont[size="1"],\nfont[size="2"],\nfont[size="3"] {\n font-size: inherit !important;\n}" >> /Applications/Mail.app/Contents/Resources/Message.css
If I come across further patterns to target, I'll update this answer to include them.
EDIT: I added attribute selectors to target font-size declarations, per @danw's suggestion. I expended his selectors to cover spaces in the style attribute's string, and to cover explicit pixel sizes below 12px.
Best Answer
There is actually an easy fix for this, as it turns out.
Abilitynet.org has a nice clear description at https://mcmw.abilitynet.org.uk/apple-os-x-10-10-yosemite-making-text-larger/
I didn't discover that source until after I'd called Apple Support, where I was given the same very simple answer, which is:
On the Apple Menu, go to System Preferences. In System Preferences, go to Displays. Change from Default to Scaled, then click the box to the left of the Default box to choose larger font or gigantic font. (Unfortunately, the Scaled display options are not available with a slider, which would have been perfect as then you could choose exactly the size that works best for you. However, the next size up from Default will probably work well for almost anyone who's been struggling to read the menu bar.)
Making this change not only fixes the visibility of the menu bar, but also carries over to other application menus. For example, I use WORD, and was having to peer at my 27-inch screen from 2 inches away to see where to navigate the cursor in order to change the font size for my document. In Google, if I wanted to read an url, I had to copy and paste it into WORD, then zoom the text display. It was VERY frustrating, and now all of that's just a bad dream.
((As an aside, as a "newbie" to the Apple world, this was my first contact with Apple Support and they earned top marks from me for response time, courtesy, and giving me precisely the info I needed. Thank you Apple Support!)