If you are reading in liberty (.lib) only, RC takes the area numbers from the cell definition in the .lib. Yes, there is generally area numbers for each cell in the .lib file.
The syntax in the .lib is like this:
cell (MYCELL) {
area : 54.316800;
Note that the above is a "unitless" number. It is up to the library vendor to define the unit.
If you are reading macro LEF files (which you absolutely should be doing), then RC will report area information using data in the LEF files. The LEF files are the physical representation of each cell, so they most defintely have area information.
Hope this helps!