According to this energystar.gov chart, the proper air conditioning BTU capacity for a 500 square foot room is 12,000 BTUs.
However, since this is an apartment divided up into several rooms with the possibility of only one A/C unit, I was wondering if 12,000 BTUs is still the correct capacity.
Here is a diagram of the apartment. The location of the A/C unit is indicated. The unit is a window style unit mounted into a wall: