While I agree with George 2. While 10x12 usually needs a 120 cfm (not all the time but a good rule).
Are we talking about metal or flex ductwork? If flex then I would install a 7 inch with a damper. Metal duct work moves more cfm then flex so if metal is chosen then a 6" might work?
Do you know the static pressure of the system. This is important as with either metal or flex it would let you know what size duct to install. Based on .5 static pressure a 7" flex duct
will move/provide roughly 120-130 cfm if installed propely. A 6"metal duct drop will provide 85-100 cfm. Either way I install a 7" with damper.
If the room gets to much air flow/cfm you can also damper it back. Without a load cal. Ran for each room then the company is guessing.
Sorry......flex duct doesn't readily come to (my) mind because we use so little of it here.
I think he said 70K is the output. When figuring 60F TR I came up with 1166 CFM!
I thought you were referring to a minimum velocity needed to help "push" the air or some other notion.
70k input @ 95% eff, divided by 1.08, then by 60F = 1026CFM.
Of course you could also use a smaller furnace and AC. AC in northern climates frequently get oversized because too high of a design temperature is used. Most of Vermont is between a 81-84F design temperature with 69F wet bulb. 1.5 tons should be plenty. 2 tons at most if there were a lot of windows and minimal shade.