And now I realize I don't really know this tradition in any depth or detail. I am astounded by this lapse of intellectual curiosity.
Until Merwin, all ballad translation used the ballad stanza or some variant of long meter, fourteener, etc...
I think of the English ballads as kind of jangly with their sing-song rhythms and full rhymes, whereas Spanish romances are noble and more metrically subtle:
Quien hubiese tal ventura
sobre las aguas del mar
como hubo el conde Arnaldos
la mañana de San Juan...
Caminante, son tus huellas
el camino, y nada más.
Caminante, no hay camino;
se hace camino al andar ...
Notice that there are two metrical patterns, both with 8 syllables or (7 in the case of the even-numbered lines here).
One is trochaic, you might say, but really more like this, with accents on the 3 and 7:
[caminante / son tus huellas].
The other is dactylic, with accents on the 1, 4, and 7: "SEHA ce ca MI noal an DAR.
There are other variations too: two mixed types: with accents on 2, 4, 7 // 2, 5, 7. There are these four kinds of line, then, but there can be variability in them. Notice: "sobre las aguas del mar." You don't put a strong stress on prepositions in Spanish, so that sound different from "Se hace camino al andar."
Take this example:
Su luna de pergamino (mixed 2, with very weak accent on 5.)
Preciosa tocando viene (mixed 2, with strong accent on 5)
por un anfibio sendero (dactylic with weak prepositional 1)
de cristales y laureles (troch. with weak prepositional 1)
The trochaic pattern tends to sound like to equal halves: de cristales / y laureles and sounds more placid. The dactylic pattern is a triple meter, with three accents rather that two:
Voces de muerte sonaron...
It sounds like a galloping horse.
And the mixed type is very energetic and polyrhythmic. So a seemingly simplistic 8 syllable form is extremely dynamic and capable of immense variations.