Two primary considerations for the OS partition:
- Leave just enough free space for expansion, temp files, installation cabs, etc.
- Don't make it so large that HD access times suffer due to large head seek travel.
Personally, my OS partition size is approximately twice the size of actual used space (15GB partition with 7 GB actually used).
If the OS partition is arbitrarily large, the HD read/write heads must travel farther to reach the used tracks of the next and subsequent partitions. For example, if my OS partition were 50GB size, the HD r/w head would have to span over 43GB of unused tracks just to reach the first track of the next partition.
Edit:
And the r/w throughput will also be faster for the track at the 15GB position than for the track at the 50GB partition.
Hope this helps!