What is the code-mandated overlap of welded wire mesh in concrete slabs?
Short version: Yes, 6” is adequate. 

Long version: (and I have a keen interest in this topic...so it will be long): Rebar is placed (installed) for TENSION. We design concrete beams and slabs to “fail” in tension so it fails with a crack, rather than designing it to fail in compression so it fails with an explosion. 

Tension occurs due to 1) bending stress, or 2) temperature stress. (Some would say “curing stress” too, but that is not correct, because the concrete is shrinking and therefore the steel is in compression. )

1) Bending stress is taken with rebar, not mesh. Mesh is too small to account for much tensile stress. 

2) Temperature stress (usually in slabs) is minor compared to bending stress and mesh can “help” absorb this stress. (Installing control joints is another way of relieving temperature stress.) 

The most accepted testing agency to identify how concrete and reinforcing steel reacts is ACI American Concrete Institute and is referenced in ALL Codes. They recommend 30 to 40 bar diameter for steel in tension, depending on the grade of steel and the grade of concrete. 

Therefore, your wire mesh (probably #6 wire) is 0.16” diameter x 30 = about 4.8”.