If this is an object you are yourself creating, it's up to you to define the semantics. As a suggestion though, getArea and getSize should be two different concepts.
- getArea should return the geometric area of the image in whatever units you deem necessary. Could be pixels, centimeters, inches.
- getSize should return the physical size of the image as it is occupied in memory.
Assuming these semantics are defined, then it becomes trivial for the class user to understand that they can use getArea() to read the location of the last pixel of the image.