Earlier today, I was using Ctrl MouseScroll -/+ to resize a PDF inside a note to make it fit on my screen to read it. A few hours later, I was looking at some text notes and it seemed that ALL my notes suddenly had much smaller text. In searching for the fix to that, I found this page... which is basically saying there's no way to change default font size on all notes, even though that's what had happened to me somehow. I remembered that I had been messing around with sizing on my PDF earlier, so I found another PDF in another note, and sized it up and down (using Ctrl MouseScroll +/- on the PDF) to see if that affected my default "normal" text in my other notes... and viola... it does. So I played around with that a little bit till I got my "normal" text notes back to my normal font size.
I'm thinking this makes no sense at all, but that's what happened. Not sure how my PDF sizing is connected to default font size in all other notes. Glad I was able to fix it though... using Ctrl +/- on every note, every time I was going to look at the note... well that was going to be a BIG problem.