https://www.imdb.com/title/tt1568150/

Based on having watched the documentary Joan Rivers: A Piece of Work and the depictions of Rivers' card index in the film and using her hands and a lateral file for scale, her cards seem to have been 3 x 5" index cards.

cross reference: https://hypothes.is/a/RvLTZjCQEe2uuaNwpTBNuA