When we're talking about opaque it's really in relation to an individual translation unit - somewhere in the binary or its linked libraries the definition has to exist for the code that uses the opaque type.
EDIT: after some more thinking I assume the key is that we wouldn't be able to have a variable of type FILE, but a pointer, whose size is always known.
> a pointer, whose size is always known
Yeah, this is exactly how it works. You work with a pointer that acts like a void* in your code, and the library with the definition is allowed to reach into the fields of that pointer. Normally you'd have a C API like
struct Op;
Op* init_op();
void free_op( Op* );
void do_something_with_op( Op* );