The Plethrum is a Greek measurement of 100 Greek feet in length (101 English feet) as well as 10,000 sq.ft. in area. It and its plural, plethra, is used in four instances in Critias (115d, 116a, 116d and 118c).
It is interesting to note that in one case (118c) Plato found the dimensions of 10,000 stade ditch surrounding the plain of Atlantis as very hard to believe but felt obliged to record it out of deference to the reputation of his source, Solon. The text states that this ditch was dug by hand to a depth of a plethrum and a stade in width.
He was forced to make this suggestion because he recognised that the dimensions for the plain of Atlantis as recorded by Plato must have been exaggerated. However, when it came to the ditch which surrounded the plain he was also contented to accept that the stade width of the ditch was excessive but that the plethrum depth was acceptable because the clay banks would have been continually subject to slippage! Feasible, but for me, not totally convincing.
(a) http://www.black-sea-atlantis.com/richter.pdf (section 7)