As noted by others, there may have been more common sizes, but not one worldwide standard. With the exception of two Japanese bookform holders, all of my holders are "typical" double-darkslide style and of U.S. origin and every company's is different. Some don't even have ribs for the rib lock, but rather slide into a pocket at end opposite the dark-slide slot. The overall sizes of the film holders vary by almost an inch in the length and width. The plate holders are closer at 3/4".
Originally Posted by Roger Hicks