You are essentially asking for deck design help. You have not stated where you are located, what codes if any you are designing to, and whether you need a permit. My suggestion is to discuss design standards with your local code enforcement official. If you don't have any codes where you are, you can safely base your design on the Prescriptive Residential Wood Deck Construction Guide (http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf
). This guide will give you allowable spans, construction details, and most everything else you need to construct a safe, code compliant deck.