In a standard design, the "foot print" is simply the amount of space the capacitor occupies on the PCB + the space required to "fit" the end wires.
As in this example the ends of that cap do fit inside the overall diameter of this vertically mounted capacitor as Boncuk stated, the foot print becomes the outer diameter of the cap to place other components around it plus the two holes within that diameter). They all have to fit on the pcb.
The "foot print" is all that you see when you place a component on the PCB (the x times y space it occupies). The "foot print" is a combination of the XY space plus the electrical connecting points for the PCB design purpose.
Think of two identical resistors mounted "one horizontally" and the other resistor mounted "vertically".. the footprint is different; in a vertical solution you have saved some xy space on the board, but you have increased the height of the whole module the PCB is designed for.
It is your choice as a designer of the PCB to choose the "foot print", you do know how the component will be mounted.
In the example of the capacitor you have mentioned, you could solder the capacitor some few milimeters off the board and still fit some components underneath the space occupied by its diameter, it would be a pretty odd design but I use it as an example to show that if you did design the PCB in this way - the "footprint" for that capacitor, would be just the two holes the capacitor does fit into..
xanadunow