Kevin,
JPG to JPG, trying to resize to fit a certain size box. It won't even let me enter my desired dimensions before it crashes.
I got your PM and will give it a try later, after I'm done editing some critical files.
I'd guess that the JPEG size estimator thread is crashing. It may be that the settings on entry to the dialog may cause the bitmap to be scaled really large, thus causing the problem.
I suggest trying the following: Quit PM. Find the Photo Mechanic preferences file and rename it. Launch PM again and then attempt your Save as.
You can find the Preferences file in the following location:
<your home folder>/Library/Preferences/com.camerabits.PhotoMechanic.plist
HTH,
-Kirk