So Im editing 1280x720 but I need to export one clip as 720x486. In my export options I select "custom" and choose 720x486 and make sure the "size to fit" option is active. Yet it will not letter box upon exporting. I have also gone into the "format options" menu and chosen my codec, the size of 720x486, and the "preserve aspect ratio using letterbox" option. Still my exported file is not letter boxed. In all instances the video is stretched vertically to fill the quicktime window and the export ignores all commands to "size to fit" and to "preserve aspect ratio using letterbox." Any ideas? Thanks. 

Nitris DX 
MC 3.1.1 
Mac 
-J 

Try sticking a 16:9 letterbox effect on your top video layer and export that (under the Reformat category)... or take your anamorphic QT into Compressor or Quicktime Pro and re-save it with a letterbox, I'm pretty sure either will let you go from 16:9 anamorphic to letterbox.

--Evan

At this time, the Avid will only resize on export, it will not reformat it as well.

You could try changing the project type to the SD-compatible version (if one exists), apply the reformat effect and export from there.  But I would take a look at the quality, because it might not be as good as using a 3rd-party app from the HD file.