I assumed the questioner was referring to 'folk ballads' in which case 'Plaisir d'Amour' or 'Strangers In the Night', as good as they might be, don't get a look in.
Personally I don't think 'a song that tells a story' is enough of an explanation. The English language tradition is basically a narrative one so virtually all the songs tell a story, even if it's only about a young woman looking for her 'Spotted Cow' and spending the afternoon snogging instead. Ballads are more than that, using techniques like incremental repetition, impersonalisation and commonplaces, stripping the narrative down to the action... a whole bundle of distinguishing features which make them unique. There are hundreds of books defining ballads and distinguishing them from the rest of the folk repertoire, among the most enjoyable (though not the most reliable) are probably Evelyn K Wells' 'The Ballad Tree' or Willa Muir's 'Living With Ballads'.
As Curmudgeon rightly says, ballads can be long or short, 'size really doesn't matter'. Personally, I've always found someone who judges a song or ballad by its length, an indication of whether or not they have the retention of a goldfish - a good ballad is as long as it needs to be.
I do think that the longer songs need a little more thought and work in order to put them across, but on the other hand, an indifferent 3 verse navel-gazing singer-songwriter piece badly sung is 3 verses too long as far as I'm concerned - let's not confuse size with quality.
Hamish Henderson described the ballads as 'The Muckle Sangs' (The Big Songs), and MacColl referred to them as being 'the high water mark of the tradition' and 'the folk equivalent of Shakespeare' . They have acted continuously as a major form of entertainment and expression for centuries, far longer than any other musical-oral form, they've been found in the mouths of poorly educated farm-workers, fishermen, servants and millworkers. Right up to 30 years ago they were still to be heard particularly from non-educated Travellers, which, to me indicates thet they must have something for them.
Jim Carroll