Approximation of ideal capacitors?
For infinite plates, the electric field between the plates is uniform everywhere regardless of the distance separating the plates.

However, if the width of the plates is much larger than the distance between the plates, the electric field is approximately uniform over much of the area and, thus, the capacitor equation approximately holds.

From the Wikipedia article "Capacitor":


  Assuming that the width of the plates is much greater than their
  separation d, the electric field near the centre of the device will be
  uniform


For an example of the corrections required when the above does not hold, see this paper:  "Form and Capacitance of Parallel-Plate Capacitors"