Air-conditioners and geysers are high-power appliances, so they need a heavier cable than lighting or general sockets. As a common guideline for Indian homes, a dedicated 4 sq mm copper circuit suits most 1.5–2 ton ACs and standard geysers, while smaller units may be fine on 2.5 sq mm and larger or long-run installations may need 6 sq mm.
These are starting points, not a substitute for a proper check. The correct size depends on the appliance's actual current rating, the conductor size and its current-carrying capacity, the length of the run (longer runs may need a size up to limit voltage drop), and the rating of the protective breaker. A licensed electrician should confirm the design.
The key safety principle is never to under-size: a cable that is too thin for a heavy appliance runs hot, ages its insulation and becomes a fire risk. For heat-prone, high-load circuits, a heat-resistant HR-FR-LSH grade is a sensible choice.
APAR offers suitable sizes in Anushakti HR-FR-LSH and Shakti FR-PVC grades — see the house wires and cables range, and how to choose the right cable for the full method.