What width should be considered to define these areas?

AMC1 ADR.OPS.B.005 aims that the aerodrome operator should ensure that assessment of the approach and departure areas is carried out which includes also cases when the aerodrome is located near a water/swampy area, or other difficult environment, or a significant portion of the approach/departure operations takes over these areas.

These areas should be defined considering, whenever possible and depending on the type of approach or departure procedure, the width of the runway strip and the published procedures envelope.