The drag is calculated by integrating the shear and pressure at the wall. The drag coefficient is then calculated by non-dimensionalizing the drag. The reference entities used in the non-dimensionalization are defined in the Reference Values panel in Fluent. Note that the "Reference Values" will not change your solution for the velocity and pressure at the cell centers. It will affect only the drag coefficient and any other non-dimensional quantities calculated from the solution.
If you have used SpaceClaim to create the geometry, refer to the steps below for Total Surface Area
Summary of Reference Values:
The following video shows how to define reference values, solution methods, residuals, and monitors. It also shows how to initialize and calculate the solution.