Offizieller Kommentar
I just texted the maximum length, and this is it:
12345679A12345679B12345679C12345679D 12345679E12345679F12345679G12345679H 12345679I12345679J12345679K12345679L 12345679M12345679N12345679O12345679P 12345679Q12345679R12345679S12345679T 12345679U12345679V12345679W12345679X 12345679Y12345679Z12345679A12345679B1234
That's 284 characters (spaces added after D, H, L, P, T, and X to allow the text to wrap instead of running off the edge of the screen!) I'm not sure where that number comes from, but that's the maximum for an M4A file (and possibly others).
I tried loading over 500 characters into the metadata in a 3rd party editor, then saved it, and reloaded the tags within rekordbox -- it truncated the rekordbox comments field at 284, but never edited the actual data field to make the contents of the file shorter, so it was still on the file.