To calculate the size of the AC you need for the best results in comfort, performance, and energy efficiency, start by calculating the square footage of your home. Once you have the square footage of your home, multiply it by 25 BTU to get ample cooling for your home. The number you get is a good ballpark figure for what capacity air conditioner you should buy.
A reputable HVAC professional like Hilo Aire will correctly calculate and explain how some of the existing variables, like climate, home exterior, sun exposure, and existing ductwork, may impact the calculations.