This is a situation that you need a professional evaluation - you need what the business referrs to as a Manual J heat gain-loss calculation. The furnace deals with the heating (heat loss) side, while the heat gain refers to the cooling requirements. It takes in account insulation, windows, orientation, etc to tell you exactly what you need
I have a feeling that the furnace output of 80 to 90,000 btu might be fine for your area, but you need a professional to check the age and efficiency.
In Pittsburg it may well be worth it to invest in a 95% AFUE furnace (it's so efficient that it vents in PVC pipe) It will save you a ton of money