The ideal size for an OS partition depends entirely on how you set up your computer. Do you have your Programs folder on the OS partition, or is it located elsewhere? Do you use System Restore? Do you use a Paging file and, if so, is it located on the OS partition, or elsewhere? How often do you empty your Recycle Bin?
I've experimented with OS partitions of varying sizes during the nearly three years I've been using XP Pro, with a variety of configurations, but my Programs (presently totalling 7.1GB) have always
been located on a separate partition ... not necessarily because it's a good idea, but because I've always done it that way.
The OS (XP Pro) partition on my 100GB HD
is presently 9.76GB, of which 5.86 are in use. The Windows folder is 2.69GB, the Paging file is 1.53GB, and Documents and Settings is .4GB, totalling 4.62GB. The remaining space is used by System Restore, the Recycler folder and a few other small files.
So, if I turned off System Restore, moved the Paging file to another partition, and emptied the Recycle Bin regularly, I could easily cut my OS partition to 4GB (as long as I kept the Programs folder on another partition).