There is a standard in the industry, but ultimately left up to the structural engineer but pretty still standard.
So for that wall (Pete correct me if I am wrong) is probably a single mat of #5 or #6 rebar. #5 rebar is 0.625 nominal inches (or 5/8" for us 'mericans) in diameter not including the deformations, the structural engineers specify the exact embedment to be a certain X's the diameter of the bar, the epoxy to be used, etc.
Since thats a house there isnt alot of loading going on that wall, if he went 6"s then its definitely going to hold up to a #6.
And the mat is 10mm bar at 16" OCEW