In ground supported slabs mesh reinforcement should be placed in the bottom of the slab between 50 and 40mm cover. Here the mesh provides and enhancement to the moment resistance of the slab.
The size of the mesh is dictated by the slab thickness and the moment capacity required (when placed in the bottom of the slab). The size of the mesh should be such that the cross sectional area of the mesh is between 0.1-0.125% of the total cross sectional area of the slab. For example in a 175mm thick slab the mesh size used in the UK would be A193.