DoctorQ is right on the money I think... with a small HDD like that there's no reason to partision, just add a works drawer or whatever on the one big system partision.
My relativly clean OS3.9 install takes 24MB, but I have the 9MB backdrops installed aswell as personal big picture and Picasso96 and a CDFS so I'd think 10-12MB is probably the minimum the install will take. If you nessesarily want to partision the HDD then make it 15MB and you'll have a little room extra for libs and other stuff.
Personaly I'd look around for a bigger HDD for the machine though. Anything 2GB+ would do it more justice and make room for some WHDLoad games so you didn't have to play floppydisk jokey. :-D
