Jeff,

Frame size will have little, if any influence on head movements when fit
properly.

The field of vision with a PAL  at near point/reading will be substantially
smaller compared to a FT bifocal. I usually recommend staying with the bifocal
and using a separate pair for the computer. Keep in mind that if the new
distance Rx is the same as the old Rx you will still have to tilt your head back
to see the screen clearly.

If you order the Comfort, the best frame size would be one that allows at least
20mm for the seg  height, and offers a subjectively adequate field of vision. If
your accustomed to a 55 x 40 lens size don't accept a 46 x 28 size, instead
moderate the change by using a 52 x 34 size. The lenses should rest very close
to the eyes with the frames geometric center about 4mm to 7mm below the pupil
center.  

Hope this helps

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
robopt@execpc.com
~~~~~~~~~~~~~~~~~~
"An expert is a person who has made all the mistakes that can be made in a very narrow field."
 - Niels Bohr

Why the Comfort as opposed to the Liberty?  I'm not an optician or
anything but my understanding is that the Liberty was specifically
designed for people who currently wear bifocals.  It's design is
supposed to be easier for bifocal wearers to adapt to.