The "Bedsheet" is actually a magazine format that became popular in the early days of the pulps. Usually, these were very large (about Life Magazine size) pulp publications, with square spines. The first true Science Fiction periodical is said to be a bedsheet (Amazing Stories, April 1926).
However, like the term "digest," there is really no exact size or style to bedsheets. In the scans above, the first three magazines are from my collection. Official Detective Stories (March 1935) is 52 pages and measures 10½" X 13". True Detective (November 1941) is 112 pages and measures 8¼" X 11". Paris Life (Jan 1951) is 50 pages and measures 10" X 12".
The last three are all from the collection of Fred Meyerriecks, and show covers from Avon publications. Fred tells me these magazines are about the size of Men's magazines in the late 1940's (note the size of the magazine the girl is holding in the last cover). It shows how Avon's book covers looked very much like their magazine cover art. Avon Love Book Monthly was also printed in digest format.