It looked like this:
|A screen-grab from a YouTube upload of "There are No Cats in America"|
ADDENDUM 3/14/16: I recently spoke with someone who was intimately involved in the production of "An American Tail". His recollection was that the shots were designed for a 1.85:1 theatrical aspect ratio. He suggested that there's a possibility that the digital transfer might have been done off a 1.33:1 master (which, itself, would have been cropped from a 1.85:1 source). If this is the case, it's pretty shameful (and shouldn't be promoted as the "original" theatrical version!) He's looking into it. I'll report back here when I know more.