The bolts they gave me with mine were about 3/8", if I remember right. I can double check later today. The size doesn't really matter because they go right through some holes in the subframe and attach to nuts which they also supply. I replaced all of mine with stainless hardware.
It was just a moment of weakness. Metric stainless hardware gets expensive! The bolts that Taliaferro supplied are 3/8-16, 1.5" long. They're Allen head, and came with two lock nuts, two regular washers, and two fender washers.