General square footage and tonnage can be estimated, however, there are a lot of other factors that should be considered when choosing the size of a unit that will perform the best. Building configuration, geography, prevailing climate, intended building use, are some of them. It is always better to rely on a trusted professional that does this regularly, and keeps up with the latest products and technology to determine the best unit for the money. Because of economics, most modern units are built fairly similar. There are not as many A/C unit component manufacturers as there once was. Most companies buy their components and assemble the unit to sell to the public. Overall quality has dropped to make their products economically attractive to contractors who are often more concerned with the bottom line as opposed to unit performance and life, that coupled with the fact that homeowner turnover is quite frequent these days.