Is it possible to have different font sizes for editing and message actually sent?
The size you set in Mail > Preferences for viewing and composing does not actually have any effect on what is seen at the other end.  To do that you have to select the font for each outgoing individually in the New Message pane (and I think also have it different than the font in Preferences).

Mail's inability to set a default outgoing font annoys many users, who sometimes then switch to Thunderbird or Entourage/Outlook instead.  Also someone did an app here:

http://messagefont.com/