The area is named "reference area". As long as it is defined by someone, you are free to choose any arbitrary value for A.
To make comparisons easier among different designs for a certain project, engineers define A and they do trade off studies with that reference. This way they can compare the performance of the wings for a certain car/plane model.
On the other hand it's also possible to Not Fix the ref area value. This would allow benchmarking various designs for their aerodynamic efficiency. (A slightly larger wing could, in itself be more efficient). The problem with this approach is that the optimization can easily break the manufacturing interface rules, size rules, weight constraints, etc.
As a result, if the thing has some physical constraints (like car-wing attachment), it's almost certain that you would define the wing area first, as a number, and not change it throughout the design life for practical purposes.