If you have a mighty mouse, you can hold down the control key and use the scroll nipple to zoom in or out...it will affect the whole screen and not just the mail program, though.
You can also turn on similar functionality for the keyboard in System Preferences > Keyboard & Mouse > Keyboard Shortcuts...under Universal Access, enable Zoom and use the key commands shown there (command option - and command option =)
In Mail...you can go View > Customize Toolbar and add the smaller/bigger tools to your toolbar...it will only decrease/increase the text in the message though and won't effect the size of listings in the mailbox area.
