How is FreeBSDs/DragonFlyBSD sendfile() system call supposed to work?

sendfile() The offset argument is of type off_t and nbytes is of type size_t while both are describing the same thing: a file length. What if my file is longer than off_t but shorter than size_t? If I'm sending asynchronously it can happen that my current file offset is larger than off_t, but I still want to send more since size_t is greater. off_t is always lower since it is signed and size_t is unsigned. I know that you can set nbytes = 0, but I explicitly want to use a length.