I am going to presume the question is referring to the message reading Window.  If so, try the following:

1. Hit ALT + H, Q1  [Message Ribbon, Zoom]
2. The Zoom Dialog opens showing 3 preset zoom percentages:  200%, 100% (selected by default), 75% along with a control box that allows you to enter or use the up/down arrows once it has focus to set a custom zoom.  There is also a checkbox for "Remember my preference," which I'd probably only check were I using some custom zoom level.
3. If you just want the 100% default, activate the OK button.

Done
--

Brian - Windows 10, 64-Bit, Version 21H2, Build 19044  

Science has become just another voice in the room; it has lost its platform.  Now, you simply declare your own truth.

       ~ Dr. Paul A. Offit, in New York Times article, How Anti-Vaccine Sentiment Took Hold in the United States, September 23, 2019