I believe "studio scale" could be anything. For example, in making the movie "Forbidden Planet", there were three different studio models of the C-57 D spaceship- the smallest was 20 inches in diameter, the mid-sized was a 44 inch diameter model, and also they built a huge 88 inch diameter ship. In addition, there was a full size partial set of the lower part of the ship used for the scenes of the crewmen standing on the ground under the ship, walking down the stair ramp, etc.
This info is from research in the recent book, "The Saucer Fleet" , by Jon Rogers and Jack Hagerty. It also contains loads of highly detailed and accurate drawings for many ships that would be invaluable to those who like to scratch-build models. Highly recommended!