An 8d would work.
Be careful what fastener you choose; the new treated lumber (last twenty years or so) is copper impregnated. you need an ACQ fastener to prevent galvanic corrosion. We have a big problem here with the fasteners disappearing and decks falling down. Not all Galvanized nails will hold up. look for the little yellow oval with ACQ in it.
There is a little info here, take a minute and read.
http://www.treeisland.com/company/news-events/what-fasteners-work-acq
