The only way to determine the correct system size (and the duct work needed to handle the air flow) is to do a heat gain / loss calculation (AKA "Manual J"). The duct work is sized by a calculation known as "Manual D". Did any of the contractors offer to perform such calculations? If not, call another contractor or two until you get one that sizes by calculation, not by 'rule of thumb'.
Originally posted by rickintampa
I've talked to three contractors so far, but their information seems confusing or contradictory. First, all three recommend a new 12 SEER unit. The say that anything rated higher won't save enough electricity to warrant the additional cost. But 12 SEER seems on the low side, especially considering the new government minimums. Also, they differ on size recommendations: One says stay with 2 1/2 ton. He claims that anything larger would necessitate replacing all the duct work. (However, he said this without ever examining the existing ducts.)