Thank you very much for posting this. I just searched for this exact problem. I just installed Safari 10 and immediately after said WTF???There's no way to change font size/minimum font size in Safari 10. They've removed that preference setting entirely. You can only change font size via overall percentages (zoom).
What an unbelievably bad omission on Apple's part.
SIMBL has been working for a while on Sierra (I got the version packaged with cDock) but recently there's been some improvements. Not only is a new SIMBL installer working, but SafariStand is working too!Add-on SafariStand is now incompatible on Safari 10/El Capitan. And in a bigger circle, SIMBL (required for SafariStand) is DOA on Sierra. SafariStand is functional on Safari 9 in El Capitan, Yosemite etc.
Among a number of features, plugin SafariStand brought favicons (available on Firefox/Chrome), modified tab widths, scrolling through tabs and dated download folders to Safari feature list. Added to the lack of font size adjustment choices in Safari 10, seems this browser continues to be less and less functional.