Well the usual size is about 12% (12.5%) of the drive. Unless u`r knowing going to have an extreme large amount of files, not files of any great size, just files by number_of, u don`t want to go any higher. Being as 12.5 is nominally used, I`d doubt u`d wan`t to make it any smaller either.
To be totally honest, I`ve never thought about any further than that.