How can I fiugure out what size system I need
Every one of the major Plumbing and Heating wholesalers in your area will have from 1 to a dozen "heating specialists" on staff. These aren't university grads or technologists or even plumbers. They're guys who've simply worked their way up from the sales counter or order desk and who've been sent on courses to teach them how to size heating and cooling equipment for new construction or building additions that will require modification to the existing heating or cooling system.
Decide what brand of A/C you want to buy, find out which wholesaler sells it, and then phone them up and ask to speak to one of their heating specialists. Tell them you'll be buying one of the brands of A/C's they sell, you don't know what contractor you'll be buying it through, and ask them what information you need to supply for them to properly size it for you. They are paid to do these calculations for heating and ventilating contractors who purchase the furnaces, boilers and air conditioning equipment they sell.