Determine maximum LUKS key file size or maximum interactive passphrase length.
You will know to check it out when you exceed the limit and see the following error message.
Maximum keyfile size exceeded.
Display compiled-in defaults to determine maximum key file size or maximum interactive passphrase length.
$ cryptsetup --help | grep keyfile\ size
Maximum keyfile size: 8192kB, Maximum interactive passphrase length 512 (characters)
Simple as that.