Stereoviews came in several sizes. The really early ones are somewhat smaller than 1/4 plate. Then in the 1880s or thereabouts, they shrank to about 1/6 plate. I've seen early stereo cameras that used a 5x7 film/glass plate divided in half for a pair of 3.5 x 5 prints, and I've seen similar sized daguerreotype stereoviews (those are rare and expensive!!!). I have a photo project pipe dream to get one of those Soviet stereo cameras that does 6x6cm negs on 120 and travel the world, re-photographing the places that are in stereoviews I own, as much as possible in the same tripod holes.