Kutools for Outlook has an option which can help you change or set the default text zoom for reading pane, and all reading/composing/replying/forwarding message windows.

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1. Click Kutools > Options.

2. In the opening Options dialog box, go to the Zoom tab, and:
(1) Check the Enable auto zoom (Percent) in read, compose and reply window option;
(2) Type a percentage into the zoom level box, or click the  button besides the box and specify a percentage from the drop down list.

Note: In Outlook 2019 and Office 365, this utility has been disabled.

3. Click the OK button.

From now on, the specified text zoom will be automatically and permanently applied to all message windows when reading, composing, replying, or forwarding email messages. See screenshot:

(1) Reading Pane: When reading messages in the Reading Pane, the email will display in the specified default zoom level automatically:

(2) Message Windows:
A. When read, compose, reply, or forward emails in the Message window, the window will show in the specified default zoom level automatically (in below screenshot, I specified the default zoom to 30%);
B. When clicking Next Items or Previous Items button to read the next or previous email in the Message window, the new message window will be zoomed to the specified zoom level automatically.