In order to get the proper sizes for a heating/cooling unit and a supply and return ductwork one would have to have completed a Manual J and a Manual D calculation sheets. These calculations will take into account the amount of windows,size of rooms, insulation r value, orientation and the climatic temperatures of the area you live in. Doing it any other way would be a foolish guess.
A lot of math goes into figuring how much supply and return is needed.
eg; a 4" diam = approx 12 sq inches.
a 5" diam = approx 20 sq inches
a 6" diam = approx 28" sq inches
That's a total 60 sq inches of area for those 3 ducts you are figuring on using in the upper floor. But since return air should be at least equal to the supply that means the 6" return you are figuring to install will fall far short of doing any real good,if any.
If you are putting supply air in that adds up to 60 sq inches then you should have at LEAST that amount in return fromthat area.
Note; DO NOT install any return in a bathroom or kitchen area.(odor)
In all, you would be wise to have a heat loss/gain and duct sizing done, then you will know for sure that any furnace(btu,s) and duct sizing will be best for you .
if you have more questions feel free to ask.