How do I change the default font size for reading in Mail 6.0?

This is a setting that can be changed in Mail Preferences. Open the dialog box by clicking:

Mail > Preferences (or Cmd + ,) > Fonts & Colors > Message Font > Select...

A dialog box should display with text options for you to customize.

I just tried this and it worked for me: http://www.pitt.edu/~hunter3/ComputerHelp/mailappfont.htm

How to permanently change the font size of the body of emails in Mac OS X's Mail program (i.e. manually get past missing font option for that reading or preview pane).

Command and A, then Command and + will increase the font size in the reading or preview pane of Mail.app, but how to make that larger size the default? If you go into the Preferences (in the Mail menu), there is a font section where you can customize font family and size for just about everything but that reading pane (crazy oversight).

First close Mail.app, then go to the Go menu (when in Finder or just click on any blank area of your desktop) and select Utilities, the double-click Terminal, then type defaults write com.apple.mail MinimumHTMLFontSize 14 then hit enter/return then reopen Mail.app to make sure the changes have been made.

Note: the default is 12, and if you want larger than 14, just replace that number with whatever font size you want.