look into data that local agencies have collected- NC state research/extension, local NRCS office, etc. you may need to adapt to permaculture concepts, but the data and equations available through these public agencies should help with sizing. nc state in particular is known for being pretty progressive (in SE) in the designed-ponds field (rainwater/retention ponds, etc.)
maybe this will help you get started
http://iswm.nctcog.org/training/Ponds_Class_082913/Ponds_handouts_combined.pdf
