PV (and solar thermal) collectors are attached to a building surface (wall, roof, etc.) or a shading surface. The area of that surface is used to determine the size of the array along with the input for fraction of surface area.

From the Input Output Reference for PhotovoltaicPerformance:Simple "Fraction of Surface Area with Active Solar Cells":

This field is the user defined fraction for the area of surface named in the parent PV object that will have active PV cells on it. The area actually covered with
solar cells will be the net area of the surface (gross area less any subsurfaces like windows) times the fraction entered here. This fraction includes the difference
between PV module area and active cells within it and any losses for how closely packed modules can be arranged on surface. The value should be between
0.0 and 1.0.

So, for example, a PV array is attached to the wall of a building which contains some windows.

The gross area of the wall is 100m2, and the area of the windows is 25m2, so the net area of the wall is 75m2.

The total area of the PV modules is 50m2, and 90% of each module is active cells, so the area of active solar cells is 0.9*50=45m2.

The value for "Fraction of Surface Area with Active Solar Cells" would be 45/75 = 0.6.