I'd consider the smaller, high-velocity ducts. This will limit you to an air handler designed for that, but in a retrofit, it makes some sense. The duct work is very small and the outlets unobtrusive.
You'd need to either do or have done a heat load analysis which will tell you the size of the a/c unit you need and how many CU FT/min of air movement you need as well. Then you can have the ducts sized.