Similar questions have been asked in the past, albeit not in the last five years, it would seem, and the Mail application has changed enough to make the answers to those older questions entirely irrelevant. For instance, there is no longer a messages.css to edit in Mail.app's resources directory and defaults write com.apple.mail MinimumHTMLFontSize 12 does not have any effect, although the strings command confirms that specific key still exists in the executable.
In Preferences, you can change the font and font size, but that's for composing email.
How can I set a minimum or default font size for viewing received email in the preview pane? I am aware that HTML email is now the de facto standard and that Mail is probably using the font size specified in the HTML tags or CSS. However, if browsers can resize a webpage and I am fairly sure Mail is using a WebView internally, then it would stand to reason that it can be done. Whether this is a user-accessible option is, of course, another matter.
Best Answer
At the top of the Mail app, there is a toolbar as shown below.
As shown above, there are buttons to increase and decrease the size of the font. If these buttons do not appear on your mail viewer you can edit the toolbar by going to the View menu item and selecting 'Customize the Toolbar'. When you click on this you get the panel shown below.
The red line on this image is directly below the icons for increasing and decreasing the font size. Adding these to your toolbar will enable you to change the font size for any mail message you view. This is same as ankiiiiiii's answer bit a different way of implementing it.
Another option which may stick is to go to Preferences under the Mail menu item. Once the Preferences pane appears, click on the 'Fonts and Colors' tab near the top of the panel. This brings open a panel where you can set the font size and font color.