Look in the QB section for starters
But to answer your question, the only essential variables for brazing performance qual that come close is base metal thickness QB-402.3, which refers you to QB-452, and overlap of the joint QB-408.1 (all references are 2010 edition, all I have with me)
There are no diameter rules for performance. Procedure qual has diameter rules.
We do a fair amount of brazing, and we add additional restrictions on max diameter for performance qual.