Peperoncini verdi o di fiume are, in the picture drawn by Regione Campania, modest green peppers with an elongated outline, only six to twelve centimetres long, and they are sold on the market. That compact, elongated silhouette is why they repay a second look: they are not a vague idea of chilli, but a named, commercially available fruit of a particular size.
The sensory note that stays with you is the scale. These are small peppers, stretched rather than round, occupying that narrow band between six and twelve centimetres. You can almost see them lying in a row: slender, green, easy to gather in the hand, more like a river of little lengths than a bulky vegetable.
The practical cue is simple and limited to what Campania records: they are commercialised under that name. If you meet peperoncini verdi o di fiume, you are looking at a marketed product whose identity rests on that small elongated form, not on extra claims the register does not make.