What size central air conditioning unit is required to cool a 2800 square foot house?

Here are two Excellent sites with engineering/scientific-based analysis of how to properly size your A/C units. Many people use "rules of thumb" that result in vastly overpowered systems, either because of lack of knowing better, or contractors intentionally trying to increase the price of the sale.

http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-289-95/

http://www.proctoreng.com/articles/bigger.html