Please follow below steps to change the default font size in Reading Pane in Outlook in Windows 10.

1. On the desktop, right click at a blank place, and select Display settings from the context menu. See screenshot:

2. Now in the popping out Settings dialog box, please click the linked text of Advanced display settings at the bottom. See screenshot:

3. Now in the new popping out dialog box, click the linked text of Advanced sizing of text and other items at the bottom. See screenshot:

4. In the coming Display dialog box, please click the linked text of set a custom scaling lever. See screenshot:

5. Please specify a percentage from the scale to this percentage of normal size drop down list, and click the OK button.

6. Now you return to the Display dialog box, please click the Apply button to save the settings.

7. The Microsoft Windows pops out and asks you for signing out. Please click the Sign out now button to sign out your account.

When you sign in your account later, you will see the font size in the reading pane in Outlook are adjusted to the specified percentage permanently.

Note: This method will change not only the default font size in Outlook Reading Pane, but also all default font sizes of all dialog boxes in your computer.

One click to change default zoom levels of Reading Pane, Message reading, composing, and replying windows in Outlook

With the Enable auto zoom (percent) in read, compose and reply window option of Kutools for Outlook, you can quickly change the default zoom levels of all message reading windows, composing message windows, replying/forwarding message windows, and Reading Pane by one click!

If you are working in Windows 7/8, you can change the default font size in Reading Pane in Outlook as follows:

1. Go to the desktop, right click a blank place, and select Personalize from the context menu. See screenshot:

2. Now the Control Panel is opening. Please click Display at the left bar. See screenshot:

3. In the new Control Panel, please check Smaller, Medium, or Large option as you need, and click the Apply button. See screenshot:

Note: In windows 8, you need to check the Let me choose one scaling lever for all my displays options to show the Smaller, Medium, Large, and Extra Large options.

4. Now the Microsoft Windows dialog box comes out and asks you for logging off. Please click the Log off now button.

When you log in your windows account later, you will see the font size in the reading pane in Outlook are adjusted permanently.

Note: This method will change not only the default font size in Outlook Reading Pane, but also all default font sizes of all dialog boxes in your computer.

If you have Kutools for Outlook installed, you can enable its auto zoom option to change the default font size in reading pane easily, no matter you are working in Windows 7, 8, 10, or others.

1. In the main interface of Outlook, click Kutools > Options.

2. In the Options dialog, go to Zoom tab, tick the Enable auto zoom (Percent) in read, compose and reply window options, specify an auto zoom percentage, and click the OK button.

From now on, when you read emails in the Reading pane, the zoom will be adjusted to the specified zoom percentage automatically. See screenshot: