The pottery was red slip ware, distinguishing it from the creamy color of the modern Qina ware common in the region, and had four small handles near the opening. The jar was also large, with dimensions roughly illustrated by Muhammad 'All as 60 cm or more in height and an opening of some 15 to 20 cm widening to some 30 cm in the flank. The jar had been closed by fitting a bowl into its mouth. Khallfah had taken this bowl with him to the home in al-Qasr where he was a servant for the Copt, Salib 'Abd al-Maslh, who preserved the bowl intact. It is Coptic red slip ware of the 4th or 5th century with a rim decorated with four fields of stripes. The diameter at the outer edge is 23.3-24.0 cm, with a diameter inside the bowl of 18.2-18.7 cm, adequate to close a mouth large enough to admit the codices, whose broadest leaves, in Codex VII, measure up to 17.5 cm. There are a few black tarlike stains about 2.0 cm from the outer edge on the under side of the rim, perhaps vestiges of a bitumen used to seal the bowl into the jar. Thus, the jar probably could not be opened readily to investigate its contents, which would explain why it was broken by its discoverers." (Biblical Archaeologist (1979), 213-4).So it was 60 centimetres tall, which is just under two feet.
However, in Elaine Pagels's The Gnostic Gospels (New York: Random House, 1979), Pagels says that the jar was "almost a meter high" (xiii), which is already a lot bigger than Robinson's estimate of "60 cm or more". One meter is 3.28ft. So the jar has grown from just under two feet to well over three feet.
Subsequently, the jar almost doubled in size. In repeated interviews and at least one publication, Elaine Pagels has stated that it was a "six-foot jar". The first example of this that I am aware of is in the 1987 video I linked to recently, a clip of Elaine Pagels teaching. It is "six feet" again in the PBS From Jesus to Christ documentary I linked to recently, from 1998. It features also in written interviews and then in print, in Beyond Belief: The Secret Gospel of Thomas (New York: Random House, 2003), 97.
A six foot jar would be huge, and is far larger than the 1.97 foot jar of the earliest accounts. I don't know where the six foot measurement came from but my guess would be that Pagels simply forgot the measurements and then, having said "six-foot jar" once, made it part of the repeated story. That can happen in story-telling. We introduce an error inadvertently, but then re-tell it and embed it in our story until we forget the origin.
I am grateful to Mike Grondin over on the Gospel of Thomas e-list for first pointing out and questioning the "six foot jar" motif from the 1987 video mentioned above, which led me to explore the different measurements of this growing jar in these other publications.