How do I calculate how many tons of A/C I need to cool a house. I don't want the contractor to rip me off

If you don't trust him why use him. If I had a dime for every home owner that told me the size unit that they need I would be rich. Its called a load calculation. It will tell you the size of heat or air you need for your area.(How many windows,R- value in the walls and attic,square feet of the home,so on and so on... )

Sorry jimmy1 for getting on the soap box. Rule of thumb and not accurate .08 to 1 cfm per square foot, depends on location.

1 ton of refrigeration will take care of 500 sq ft. with 8 ft ceilings