Star Wars effects were done in VistaVision, they considered using 65mm but went with VistaVision. Of course the live action footage was shot 2:35:1 with anamorphic lenses on panavision cameras using regular 35mm film.
From what i understand vistavision is 35mm on its side for a larger negative area for the composite optical effects.
When ILM was founded Paramount was throwing out all its Vista Vision equipment so they bought up a whole lot of cameras and other equipment for next to nothing. They then built their own custom cameras and optical printers from the parts. VistaVision was developed for Paramount in the 1950's as a rival to Cinemascope and other widescreen processes. Its basically 35mm film run sideways through the camera so each "frame" is essentially two 35mm frames side by side effectively doubling the frame size. This also elininated the need for bulky anamorphic lenses and the associated distortion problems of the early cinemascope lenses. And because it used ordinary 35mm film it was cheaper to use than 65mm.
The theory of using this for optical effects composites is that while different generations of VistaVision film would degrade, you'd elminate some of that degradation when you shrank the final output to 35mm - this would "hide" the extra grain and sharpen the image up.