There is one aspect of this we can define, and that's the structure of the poetic form called a ballad. Like a limerick or a sonnet, this does have rules, and these feed through to many musical ballads too.
A ballad poem, in its standard form, is constructed from quatrains of alternate cross-rhymed tetrameter and trimeter. In other words, it's generally a four-line verse, usually alternating between four and three beats to a line, with those beats falling on the even-numbered syllables of each line.
That's the ballad in its most archtypal form, and if you want to call it an eight-syllable/six-syllable structure, then good luck to you. As someone here's already said, though, it's more useful to count the beats in each line rather than the syllables.
Variations to that structure, such as the six-line verses Oscar Wilde uses in The Ballad of Reading Gaol, are acceptable, but if you step too far from the ballad's basic rules - at least in poetic terms - then what you've written won't, strictly speaking, be a ballad at all.
In its popular usage, of course, "ballad" just means anything that can vaguely be described as a story song or (sometimes) any song that's not an outright rocker. At that point, though, all useful definitions break down and the word just means whatever the speaker wants it to mean.