Since the statement is emphasizing surface area, and the area is so small, I would say:

In Charro Corto, five out of nine (56.9%) areas planted in corn are only a manzana* or less.

*I don't know how you are translating "manzana".

Plenty of references to "areas planted in corn:"

agroclimatic evaluation of the soviet corn program - Central ...www.cia.gov › library › readingroom › docs
PDF
In 1957, **areas planted in corn amounted to only** 18.3 million hectares (45.2 million acres) 46, a sizable cutback from the preceding year, * This reduction was ...

Where is the USA Corn Belt, and how is it changing? - UNL ...digitalcommons.unl.edu › cgi › viewcontentPDF
1. Map a recent (2010–2016) geospatial example of the Corn Belt (**areas planted in corn above selected thresholds**) and demonstrate its space-time dynamics,
by TR Green - ‎2017

Farms is indeed the stock translation for the first part, but agricultural holdings also works for smaller plantations.

Also, on the Brit. Isles, maíz, despite sharp-fanged translators and interpreters - whose grand/children may have read Maisy's train - tucking into 'corn on the cob', the produce or product sold in called in fact 'maize'

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Note added at 7 heures (2020-07-04 09:37:52 GMT)
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smaller plantations: una superficie de una manzana o menos....

https://www.whitetailhabitatsolutions.com/blog/how-many-acre...

or 'areas/plots dedicated to (growing) corn' may be options that move away from the connotations and subtext of the 'plantation' as such, which may be precisely the author's intent here in using 'explotaciones' and not 'plantaciones' or another term that might be frequently associated with greater production and agro-industry.

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Note added at 1 hr (2020-07-04 03:44:34 GMT)
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After thinking more about the possible nuanced meaning or intent by the author to convey something slightly different from plantaciones (the use of which the author may have very purposefully avoided), perhaps my second idea may be preferable, i.e., something along the lines of "areas/plots dedicated to (growing) corn."

...five out of nine plots dedicated to (growing) corn have an area of xxxx

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Note added at 1 hr (2020-07-04 03:47:41 GMT)
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Or if raising the register just a little bit may be seen as advisable, 'plots/areas dedicated to cultivating/the cultivation of corn' may also be useful options.

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Note added at 2 days 14 hrs (2020-07-06 16:51:44 GMT)
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Or perhaps, 'plots where corn is cultivated'

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Note added at 2 days 14 hrs (2020-07-06 17:08:44 GMT)
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And the idea about the author's decision to not use 'plantaciones' may also apply to his or her choice of not using 'granja' either. Efectivamente, un término como 'explotaciones' podría referirse a 'sembríos', 'huertas' o incluso 'conucos', si se usa esta última palabra en Nicaragua. Y en esto, 'o menos' puede ser la clave.