Tim, I haven't done the calculations in years but its not just square footage. Its amount of insulation, glass size and insulating value, sun exposure, etc. A lot of things go into the calculation.
Use the total load to size the system. Use the room load to size the ducts and piping. You can find the tables for what the various size round duct piping and registers allow. 6" is around 105 cfm give or take 5 cfm. You have to match the registers. The home centers may have a chart.
The trick is in figuring the size of the square ducting coming off the air handler. For long runs you may have to reduce the size as you get close to the end depending upon the number of ducts. To get even distribution at all the ducts, you need to maintain an even pressure going to the vents. You can install manual round duct valves off of the main duct to tweak the system a little. A multiple thermostat system uses electric valves to open and close the ducts as needed for air delivery.
There are some general rules of thumb for different areas of the country that apply to a new tight energy efficient house based on the size. Your needs may be more or less depending upon your house. Try some of the HVAC forums.
I forget your original post but you size separately for the furnace to get the air handler and heating needs. Then size the tonnage of the AC based upon the total load. When you have a size for the furnace, bigger may be better. When you have size for the AC, bigger may NOT be better if you are between sizes. Depending upon your location, the bigger AC won't run as long which reduces your comfort level. For example if you calculate needing 33000 btu of cooling, that puts you half way between 30,000 (2 1/2 ton) and 36,000 (3 ton). The best option may be to go with the 2 1/2 ton AC.
If all else fails, you don't have to reinvent the wheel for HVAC sizing. Go look at comparable houses to get some ideas.
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