The stereo format is the size of a stereoview. The original format of early stereoviews was 8,5x17cm. This size is related to the first manufactured Brewster-style stereoscopes and thus became an unofficial standard format. Daguerreotypes, glass stereoviews and paper card stereoviews were manufactured in this leading format until the early 1890s. This format remained the leading format for paper card stereoviews, but more compact formats emerged for glass stereoviews.
Jules Richard introduced the 45x107mm format in 1893, later followed by the 6x13cm and 7x13cm glass formats. The 7x13cm format was not very successful, but 45x107mm and 6x13cm became the leading formats for stereo photography until c.1930.
There were many other formats that were less successful.