The schoinos is a land measure and a term used frequently in the geographers. The Suda here calls it an "agricultural" measurement. This perhaps shows the influence of Herodotus 2.6.2, which contrasts various measurements by describing those used by people with different quantities of land; those that measure in schoinoi are to be contrasted with those lacking in land (gewpei=nai), which may have conjured up an agricultural scenario. Alternatively [DW] the Suda contributor -- followed in the eleventh-century Etymologicum Gudianum -- has simply miscopied or misremembered the entry in Photius' Lexicon and the Synagoge, where the gloss is me/tron gewmetriko/n; references at Photius sigma916 Theodoridis. See also sigma 1811.
In 2.6.3 Herodotus states that it is an Egyptian measurement, although Pliny says that it is Persian (Natural History 6.30.124). Herodotus also records that it is twice the length of a parasang, but it appears that in the ancient world the length varied considerably. For discussion of the evidence for the length of the schoinos, see P.M. Fraser, Cities of Alexander the Great (Oxford: Oxford University Press, 1996) 76-77 n.2.
For quite another meaning of the headword see sigma 1813, sigma 1814, and generally LSJ s.v. (web address 1).