1. Go to Mail > Preferences
2. Click on Signatures
3. Select the offending signature
4. Highlight the text that is mysteriously enlarged by Mac Mail (ie probably the entire signature)
5. Right-click (bottom-right side of trackpad) and go to Font > Show fonts or select these from the menu bar. (but don't forget to highlight the text)
6. Now set the size to your desired size. Mine was set at 13 but should be 12.
7. When you now open a new mail, NOTHING HAS CHANGED. This is normal.
8. Send a test mail, for instance to your own gmail acccount.
Now the size should have been adjusted. I have found that there is no way to 'apply' or 'confirm' the changes. Just select a new size and close the inspector window. Hope this helps!
