Need Help..

I read on EHow or something like that that an A/C duct minimum size is 8". I'm wondering where this code reference is from. Does that mean you can't run an 8" duct and branch it into two 6" ducts for a room?

I currently am using a hydronic system to heat my home. Recently, I purchased a furnace, as well as a central a/c unit and have a lot of questions. To start, the house is roughly 1900 sq ft. this does not include the basement, which is approx 1200 sq ft, but is not finished. I would like to install the ductwork myself, as there is none since a hydronic system is used for heating, and hire a professional to set equipment charge the a/c etc. Where could I go to get a layout for my ductwork? There are a lot of software calculators out there, but they seem quite complex. Any input is greatly appreciated.