Predating photography, the Dutch automated paper-making and became a major supplier of paper products. The paper making machines outputted a large sheet of paper that was sized to the span of a typical workman’s outstretched arms. This sheet was then cut down to make various sizes of writing and drawing paper. The Dutch marketed an 8 X 10 drawing paper that was very popular in England. The 8 X 10 inch was typically cut in quarters to make 4 X 5 sheets, and these were cut down to make 3 ½ X 5 inch sheets. The 8 X 10 was also cut in half making two 5 X 8 inch sheets. These were trimmed to 5 X 7 inch, a more pleasing aspect ratio. You should research the “Golden Rectangle” an aspect ratio handed down by pre-renaissance artists. Photo prints sizes naturally followed these popular artist formats.
The first practical photographic process was the Daguerreotype, an image on metal plates. Initially the camera accepted a plate that measured 6 ½ X 8 ½ inches (165 X 216 mm). The camera back could be re-positioned to allow partial plate imaging. The sizes offered were: Half Plate 4 ½ X 5 ½ inches (114 X 130mm), Quarter Plate 3 ¼ X 4 ¼ inches (83 X 108mm), Sixth Plate 2 ¾ X 3 ¾ inches (70 X 83mm), Ninth Plate 2 X 2 ½ inches (51 X 64mm). These sizes became the international standard for photo prints and picture frames.
In the early 1900’s, Thomas Edison was working on a motion picture and viewing system for Penny Arcades. His team, headed by W K L Dickson and William Heise, bargained with George Eastman (Kodak) for film. Kodak was making 70mm wide long rolls for the Brownie camera. The Edison team purchased the 70mm long rolls and slit them into two rolls of 35mm. The motion picture camera required that the film had edge perforation to engage sprockets teeth. These smoothly transported the moving film in the camera. The sprocket holes were spaced on both edges of the film. The space between for an image was 24mm. The Edson team set the format at 24mm long 18mm height. This became the initial motion picture aspect ratio.
In 1913, Ernst Leitz tasked his chief engineer Oskar Barnack to invent a still camera to accept the now popular 35mm socketed film. The camera was to held primarily in the horizontal position to Barnack set the frame length twice the 18mm = 36mm and retained the 24mm as the image height. The result was a frame that measured 24mm height by 36mm length. This was the Leica marketed 1924.
Also, keep in mind that for most of photographic history, the final output was a paper print made from a semitransparent negative. The film needed to we wider than the actual image size as this lip or opening was needed supported the film during exposure. Likewise, a negative gate was required to support the negative by the edges during the printing exposure. The camera’s film mask and the enlarger negative mask extended past the image, it cut off about 2% of the frame, both top and bottom and sides.
That’s just some of the rest of the story regarding popular photo image sizes. .