It all depends on the reasons why you partition in the first place. Long ago, you would gain a performance increase with smaller partitions due to the read swipe on the heads across the platter. However, today's disks have better cache and higher rpms, so its no longer an issue. If you partition based upon getting lost in your directory structure, you have to determine how much growth you'll have with your music partition, versus your movies, versus your programs.
IMO, a quick way to partition would be 1 for OS, programs, etc. (things that you would have to reinstall if you had a disk error) and a 2nd for music/movies, etc. (things that the structure wouldn't change even after a new OS install) at about 100G/400G.