Every heating/cooling appliance must be properly sized to the load. The load is properly determined by performing a Manual 'J' heat load. Furnaces with higher cooling loads will require higher blower speeds - while in cooling mode - but they make furnaces with multiple speeds; one low fan-on, one heating; a bit higher, and one for cooling; typically the highest speed. Then of course there are variable speed ECM fans burners or less expense two-stage furnaces.

Once you have the heat load the duct work can be addressed. It does pay to look at the whole system before starting. There is no excuse for installing heating equipment more than 150% of the heat load or for that matter installing any piece of heating equipment without performing a manual 'J' first. This is how the homeowner should determine which furnace to choose.

It is the dealer not the furnace.