cuartillo.

(a loanword from Spanish)

Headword: 
cuartillo.
Principal English Translation: 

means one-fourth of an almud or a real
(a loanword from Spanish)

The Testaments of Culhuacan, eds. S. L. Cline and Miguel León-Portilla (Los Angeles: UCLA Latin American Center Publications, 1984), 13.

Attestations from sources in English: 

yteh Calaqui Maquili quartillittos tlauli = in which 5 little cuartillos of maize fit (Calimaya, Toluca Valley, 1762)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 222.

calaqui tlaoli tuctli hume almo Yhua Yei Cuartilio Yhuan tlacCu Cuartilio = where 2 almudes and 3 ½ cuartillos of maize seed fit (Santa María de la Asunción, Toluca Valley, 1783)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 187.

Charles Gibson, Aztecs under Spanish Rule (p. 311), gives a "cuartillo" to be 1/48 fanega (0.18 acre). A cuartilla, in contrast, was a fourth of an arroba or a fanega, much bigger.
James Lockhart collection, notes in the file "Land and Economy."