Quick answer
As a rough guide for 230 V single-phase home circuits: a 1.5 sq mm wire suits lighting and light loads up to roughly 3,000 W; 2.5 sq mm suits general power sockets up to roughly 4,500 W; and 4 sq mm handles heavier appliances up to roughly 6,000 W. Larger loads need 6 sq mm or more.
Treat these as starting points, not exact limits. The true safe figure — the cable's ampacity — depends on the run length, how the cable is installed, grouping and ambient temperature, and it must be matched to the right MCB. See also what conductor sizes mean.
Under-sizing causes overheating and fire risk, so when in doubt, size up and have a licensed electrician confirm. Browse the right sizes in APAR house wires and cables.