A volume is called a "LUKS volume" because it has a LUKS header. Thus if you convert a non-LUKS volume into a LUKS volume then you do get an additional header and do lose data space.
The LUKS header can be on a differenct device (
--header) but I do not know whether
cryptsetup-reencrypt supports that. But most probably you want to have the LUKS header within the RAID anyway.
Thus you have to
reduce the file system size by at least 4MiB
I suggest that you decrease the file system size by a bit more than the value for
--reduce-device-size (which I guess must be 4MiB or more).
You may want to overwrite the gap between the encrypted LUKS data and the end of the device with random data afterwards. But be really careful with that. You should first make a backup (to a different volume, of course) of the blocks you are going to overwrite.