I don't think there's anything more to it than this: "ladrillos gordos" are thick bricks and "ladrillos grandes" are large bricks. I have not seen a more technical term in English documents of the period.

In the Romero article, "ladrillos gordos" is the term used in the original document ("cuatro mil seiscientos y veinte reales que importan sesenta millares de ladrillos gordos de labor"), and I imagine this is true of your text too. Gordo/thick simply means thicker than standard, and standard thickness at this time was appreciably thinner than now, though the bricks were often also longer and wider, as can be seen in buildings of the period.

According to the first RAE dictionary, the Diccionario de Autoridades (1734), a "ladrillo" was a "pedazo de tierra amassado y cozido, de un pie [27.86 cm] de largo y algo menos de ancho, de tres dedos [5.22 cm] de gruesso, que sirve para las fábricas de casas, murallas y otras cosas".

Only about fifteen years before this, Teodoro Ardemans, municipal architect of Madrid, stipulates a thinner standard size for bricks. Ardemans says in his Gobierno político de las fábricas (1719) that "la gradilla para cortar el ladrillo haya de tener diecisiete dedos de largo, trece de ancho y tres y medio de grueso", which means that the bricks themselves would have been "un pie [27.86 cm] de largo, una cuarta [20.90 cm] de ancho y dos dedos [3.48] de grueso". Another source gives the measurements of "ladrillos toscos" as "doce dedos [20.88] de longitud y cuatro, cinco o cinco y medio [6.96—9.57 cm] de anchura y dos [3.48] de grosor".
http://books.google.es/books?id=IDpSG99f8EMC&pg=PA662&lpg=PA...

As time went on, standard brick thicknesses slightly increased, but the hand-made bricks in use up to the nineteenth century in Spain were between 3 and 5 cm thick: thinner than today.
http://www.albaplataenequal.org/doc/doc135.pdf

In England, by the way, bricks were somewhat thicker; brick sizes were first regulated in 1571, when the standard dimensions were set at 9 inches x 4.5 inches x 2.25 inches (22.9 x 11.4 x 5.7 cm). In 1766 they changed to 8.5 x 4 x 2.5 inches (21.6 x 10.2 x 6.4 cm).
http://www.howto.co.uk/property/research-house-history/physi...

So "ladrillos gordos" just means "thick bricks": bricks that are thicker than the standard size. "Ladrillos grandes" presumably means bricks that were larger than standard in length and/or width, and probably in thickness too.


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Note added at 2 hrs (2012-02-26 23:00:36 GMT)
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In this historical context I think it would be better just to call them "thick" rather than to use any of the modern terms for brick sizes. Since the standard size was thin by our standards, these "ladrillos gordos" would not have been very thick by modern standards.

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Note added at 2 hrs (2012-02-26 23:18:56 GMT)
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Thin bricks were the norm because of the difficults of drying thick ones before modern kilns:

"Even when mixed with river sand it seems unlikely that thick bricks would be easy to make with this material using hand-moulding
techniques."
British Brick Society Information, no. 39 (May 1986)
http://www.arct.cam.ac.uk/personal-page/james/bbs/bbs_39.pdf

"thick bricks ehg027.4 96010 1"
http://andronikos.kyklos.co.uk/Anno_CRM.php?id=3251

"It stands on a high abrupt hill called Monte Deci, where are
four or five mean cottages and a little church, and a building of thick bricks or tiles, about twenty-five paces in circumference"
The Journal of Montaigne's Travels in Italy, p. 17
http://scans.library.utoronto.ca/pdf/1/27/journalofmontaig03...

"A favourite string-course in buildings of the second period is constructed of thick bricks set diagonally between two thin courses"
F. M. Simpson, A History of Architectural Development (1905), I, p. 219
http://home.us.archive.org/stream/ahistoryarchite02simpgoog#...