"Boleos" are stones that are larger than gravel or pebbles but smaller than boulders. The proper geological term for stones of this size is cobbles.

"Se rellenan los pozos con boleos de 1½" a 5" de diámetro sin arena"
http://www.corporativofloga.com/verdecreto/docs/sistema cons...

"Boleos: partículas de roca menores que 12” (300 mm), pero mayores que 3” (75 mm)"
http://www.constructorcivil.org/2010/01/tipos-de-suelos-segu...

">256 mm / >10.1 in / Boulder
64–256 mm / 2.5–10.1 in / Cobble
32–64 mm / 1.26–2.5 in / Pebble"
http://en.wikipedia.org/wiki/Particle_size_(grain_size)

"Note that Cobble is a generic geological term for any stone having dimensions between 2.5–10 inches."
http://en.wikipedia.org/wiki/Cobbles

An alternative would be "large pebbles" or "small boulders". Strictly pebbles go up to 2.5 inches (6.4 cm). "Boleos" can be this size, but they are often quite a lot larger. However, even the largest "boleos" are smaller than all but the smallest boulders, which are at least 10 inches in diameter and usually much larger.

"Compared with the distribution of physical factors in the study area, the fish selected shallow habitats with the riverbed covered by cobbles and boulders. Boulders and larger cobbles were chosen by larger individuals as a habitat."
http://www.springerlink.com/content/7m5h0b6wg4hwkmvq/

"“Pebble clusters” are characteristic of some coarse, poorly-sorted fluvial deposits; they form when pebbles in the course of transport are stopped by cobbles or boulders protruding from the river bed."
http://www.sciencedirect.com/science/article/pii/00370738689...

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Note added at 53 mins (2011-06-16 21:57:04 GMT)
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Boulders in Spanish are simply "rocas" or "bloques" (>25 cm):
http://es.wikipedia.org/wiki/Bloque_(roca)