Load calcs for room addition
There are a lot of factors to be discussed for the proper load calc in your specific addition. Call a local contractor, or purchase some load calc software.
There is some software that will ask you all of the questions needed about your addition to tell you the proper load calc. But for the system to work properly, you will need all of the ductwork to be sized correctly to acommadate your addition.
I would definately suggest calling a contractor to come out to install everything properly, or you will either have a lot of problems, or will be spending quite a bit of time learning what takes most people a few years to do properly by themselves.