If you have 10.2.8 or earlier, your problem might be with the automatic printing scaling 'feature' in which case simply narrowing the Mail window should fix it.
If you have a later version of OSX then your problem might be a corrupt printing preference file. Do the images scale down accordingly? If you save it to a PDF does the small print carry through? The preference files I'm talking about all start with com.apple.print and finish with plist. At a guess, dragging these to the Desktop and restarting should fix your old and weary problem.
