Text size suddenly got bigger

After a break I returned to my work and began typing. But the text size has grown bigger! There’s no one else around and the place isn’t haunted. Is there some way I could have done this by accident? Even more curious is that when I went to FONTS to change the new Optima Regular 24 point text back to the original size, the next size down is 18 point, which is SMALLER than what I’ve been using. Any idea what is going on?

After posting the above message I went back to FONTS and saw that in Recently Used fonts a Japanese font had been used. I have a Japanese keyboard and now I remember accidently hitting the Japanese conversion input key and typing a couple of strokes before reverting back to English. This is obviously the cause of my problem.
Now my question is: When I accidently hit the Japanese conversion key again (for I will from time to time) is there any quick remedy to stop the English font from changing in size when I revert back to English?