After the invention of photography (1839: Niépce / Daguerre) stereoscopic photographs were soon realised with the several processes.

The first stereoscopic photographs were uniques corresponding to the developed processes: daguerrotypies, talbotypies. After the invention of the wet-plate and the dry-plate proocesses in the fifties of the 19th century it mostly had been paper prints pulled up on pasteboard until the twenties of the 20th century.

Here you can see some sizes of stereoscopic views.
Paper prints pulled up on pasteboard [a] [b] were the wide-spread format.
Next there were glas-stereos [c], printed stereocards [d] and since about 1939 the viewmaster size [e]. Then there were special sizes, based on the standard small filmsize, like the size of the Koak Stereo Camera [f] of 1954.

Soon every size needs another kind of viewer.

[ a ] : Johnstown, PA, after the flood;
The Universal Steroscopic View Corporation, not dated,
pasteboard size 8,3 cm x 17,2 cm, picture size 8,7 cm x 14,5 cm
[ b ] : Crystal Palace, 22. Exterior of the Palace, from the Grand Terraces ;
not dated, about 1865;
pasteboard size 8,3 cm x 17,2 cm , picture size 7,8 cm x 15,1 cm ,
[ c ] : Versailles - Signature de la Paix 28 Juin 1919 ;
Stereo-Glaspositiv, Verascope Richard
Glasgrösse 4,5 cm x 10,7 cm
Einzelbildgrösse: 4,0 cm x 4,0 cm
[ d ] : S.E. across Lagoon to Electrical Bldg. And Enchanted Isle
– Chicago Expostion 1933
picture size 5,5 cm x 11,9 cm
[ e ] : Kodak Stereo Camera size, 1954
size 3,0 cm x 10,0 cm
single picture size: 2,2 cm x 2,5 cm
[ f ] : No. 156 New York City; Viewmaster reel, 1952, 7 stereoviews,
diameter 9,0 cm
single picture size: 1,0 cm x 1,2 cm