[OS X Emacs] fit-frame resizes frames too large in Aquamacs

[siemsen at ucar.edu]

I'm using Aquamacs "2.0preview2".

When I open a file, I'd like the frame to automatically resize to fit  
the text, as much as possible.  I found fit-frame on the web, which  
seems exactly what I want.  I discovered that it's distributed with  
Aquamacs.  When I did M-x fit-frame, it resized the frame.  Very cool!

Trouble is, it makes the frame wider than needed.  I think this is  
because I've customized Aquamacs to use a default font height of  
"120", while fit-frame uses the height attribute of the "default"  
face, which is 130.  In other words, in my customizations.el file I have

(custom-set-faces
  '(autoface-default ((((type ns)) (:inherit default :height  
120 :family "Monaco" :background "#02053c"))))

...but when I evaluate (face-attribute `default :height) I get 130.  I  
think fit-frame is using 130, hence my frames are resized too large.   
Admittedly, I'm guessing a bit here.

To try to fix it, I put this in my Preferences.el:

(set-face-attribute `default nil :height 120)

It didn't work.  It doesn't set the value used by fit-frame.  Instead,  
it seems to set the height of the font of the current frame, not of  
the height of the default face.

Am I in the right ballpark?

I have a second problem getting fit-frame to run every time I open a  
file, but it's not related to this problem, so I'll post a separate  
message about it.

-- Pete