We use 5/64" metalcore for flat welds. I had about 10 years or so welding experiance in one of my past lives.
My hip-shot response would be "as small as the welding operator can make it and still maintain proper profile, penetration and not have excessive porosity".
Beyond that, when we get into a tricky situation, we run some test welds, bend them and send out for x-rays.
Did you try the welding forum of Engineers Tips?