Illustrative Context

The canyon is wide here, but further down it’s narrow.

Target Sense

• The basic antonym of wide (i.e. of gaps, spaces), not the opposite of broad (i.e. of objects). That is, the lexeme entered should be the normal adjective to refer to a small (horizontal) gap or space between two sides, rather than the small (horizontal) breadth of an object or person.
• If your language has a cover term for both, then as a basic, general term that is indeed appropriate; but if there are different terms, select the more default one for narrow gaps, not thin objects or people.
• The target lexeme in English is therefore narrow, not thin.
• Avoid intensifying terms, or those that are not neutral but primarily entail a quality judgement as (excessively) tight, (too) close a fit, e.g. of clothing.