Q: font size

The MacOS does not use dynamically sized fonts, they are static which means that as the resolution of Mac displays have increased dramatically the system fonts have appeared to shrink. When the retina displays were first introduced Apple provided a second set of static fonts so text didn't shrink into the unreadable zone but you can see (or not see) the results.

For system fonts such as the menubar and dialogs you have only one recourse: set the screen resolution smaller. Depending on your Mac and visual tolerance this may or may not make things better. (My retina MBP looks good to me at a lower resolution but my MBA doesn't.) For programs you can sometimes control font sizes but it varies from program to program.

Once upon a time Apple was said to be working on a dynamically sized graphic OS but that was years ago and there appears to be nothing on the horizon. Sadly, Windows 10 does a very nice job of dealing with this and doubly sadly, as my eyesight worsens with age I can imagine the day when I may be forced to move to Windows for this reason.

I too find some font sizes too small, especially in some email that occasionally is just too small to read (strangely most other email is easy to read [?] ). I have not tried changing screen resolution or system font sizing, but instead usually do one of two things... Most commonly I hold down the control key and scroll with the mouse to increase / decrease the entire screen (you will need to be sure scroll/zoom is enabled in the Accessibility section of System Preferences as shown by the second check mark in the image below). In the case of email or web pages that are too hard to read I select the message or the web page so it is the active window, then hit the command key and the + key to increase the font size to a readable size.