Zahir: identifies what meaning first becomes apparent from the wording.
Nass: identifies the point the utterance was specially brought to establish.
In the verse concerning sale, the immediate meaning is sale is lawful and riba unlawful. The particular purpose is to reject treating the two as legally equivalent.
The distinction does not depend on sentence length. The added force of nass comes from a clearly disclosed communicative purpose, not merely from a more emphatic sound or a longer statement.