Using New Outlook on Windows for professional communication and productivity
I don't have a "reading pane" that opens when I double-click it...I'm talking about the website, not the app.
I am also using Outlook in a web browser and not the Outlook app. I do have the reading pane enabled, though.
So, I am trying to figure out what you are seeing on your screen in hopes that I can help. Are you looking at an individual email in a separate window like below? If so, try clicking the Zoom option at the top (circled in red below) and set it to 100%.
If you don't see a Zoom option, please post a screen capture of what you are seeing.