1.) You have not created an extern
array. Rather, you have declared that an array exists, and it was created somewhere else (externally). And where it was created, it does have a specific size.
2.) Because you have not created an array, no size is allocated for it. (see #1). Rather, the array was created in another file/module/component (externally), and this is a declaration that the array exists, and may be accessed here.