Did it change the font size only in your email, or across everything you go to?
Also, what email are you using? They usually have settings within the page for the email client.
A temp fix (if it’s just email) is to use the Ctrl-(+ or -) to size in and out, or you can press Ctrl and use your scroll wheel on the mouse.
I’m using Outlook online, and it changed that one page. Every otehr page ws the right size anyway.
I’m using Outlook online, and it changed that one page. Every other page was the right size anyway.