To change the view for all items:
Click Tools > Options.
Double-click Environment.
Click the Views tab.
Make selections in the Default Compose View & Font and Default Read View & Font group boxes.
(Optional) In the Default Read View & Font group box, select Force view next to Plain Text if you want to prevent HTML-only messages from being displayed when no plain text version is available.
If you select Force view, a message informs you whenever an HTML-only message cannot be displayed; however, you can still click View > HTML to view it. If you do not select Force view, HTML-only messages are displayed in HTML, even though you have selected Plain Text for the default read view.
Select a default font style.
Select a default font size.
Click OK.
To change the view in one item:
Open an item.
Click View, and then click Plain Text or HTML.