Thank you!!! It worked perfectly - the cursor wasn't actually blinking, just a grey box, but I entered the text and hoped for the best. Thank you also for your detailed, step-by-step instructions; I actually did know how to copy and paste, but some on this forum (or their relatives) might not, and I didn't know any of the rest. Besides, it's always better to include information people may already know, than to assume they know things which they don't.
Now the only issue is the one posed by poing99: "Given that it works using terminal it suggests the functionality is still there but Apple have removed the menu option for some stupid reason." Why make users open an application, open a utility, copy and paste text, and restart Safari, instead of just clicking on a menu option? If you have a lot of tabs open in Safari, and a mediocre broadband connection, a relaunch can really slow you down. If Safari 10.0, for some reason, is designed not to process a font size change unless you relaunch it, why not just leave the menu option in place, and give you a "Relaunch Safari?" box if you reset the font?
I am hesitant to click on the "This solved my question" box for Kaeve's original message, because (at least in my case) you ought to share the credit. I also worry that Apple, seeing a "solved" flag, will just ignore the whole issue and not even bother working on a fix.