Parallel view images require a stereo viewer like an antique stereoscope or a modern stereo viewer to view in three dimensions. They can also be viewed without a viewer by focusing one’s eyes into the distance, beyond the image. Parallel view images can not be wider than the distance between one’s eyes — about five inches or 125 mm.
Cross view images can be viewed in three dimensions by focusing one’s eyes in front of the image by slightly crossing one’s eyes. There is no limit on their size.
Disclaimer
Some people can see 3-D in parallel view, but not in cross view; some people can only see 3-D in cross view; some people can see 3-D with both methods; and some people can’t see 3-D with either. It may take some practice, but one or the other method should work for most people. Also, it is a myth that one’s eyes can get stuck cross-eyed.