I just installed Safari 10, and I'm not really sure if the font size has gotten smaller or not, but it seems like I'm now seeing a brighter screen (Is it possible?), and if the font size is smaller it's unnoticeable to me.
I poked around a bit to see if Apple has moved the font size pref, and I didn't find it.
I now see that Safari's default font size
has gotten smaller; the FTM Forums page takes up less space on my screen.