My rule of thumb is to always separate the OS from the rest of the partitions. If I had a 28gb HD I would make about 5gb for installing the operating systems, then maybe 8gb for other programmes and the last 15gb could handle the music. Basically by giving the OS it's own partition, you'll make life a lot easier for yourself when it comes to defragging and re-installing the OS. Saves you having to reformat a whole 28gb.
It's really a trial and error thing and you just have to plan ahead a little. Realise how much music you have and make the partition the appropriate size to fit, remembering you may add more in the future.
Take into consideration what programmes you are going to install as well and again, size the drive accordingly.
Installing 98 - click here;EN-US;221829
Dual Booting - click here
Hope that helps.