You could have gone larger, but it doesn't really matter much.
The biggest effect of the allocation size is that every file will be rounded to a multiple of that. So 4k is a pretty good trade off. If you were storing millions of files less than one 1k, then the 512 byte size would make sense. If all the files you were storing were gigabytes in size, then the 64k would probably gain a little extra speed.
The default is good.
Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs