[dm-crypt] Maximum useful size of keyfile

What is the maximum useful size of a keyfile with respect to the --cipher
and --key-size parameters?

For example, with `--cipher aes-xts-plain64 --key-size 512` is there any
advantage, be it security or otherwise, of using a keyfile which is larger
than 512 bits (64 bytes)?

The context of this question is that I am creating an encrypted disk that
will only be unlocked with keyfiles. I noticed that `cryptsetup --help`
gives a maximum keyfile size of 8192kB which is quite large in comparison
to the key size. I did not find the answer in the FAQ.