it depends ...
I assume you want to install XP on c:.
C: should be as big as needed and as small as possible.
(speed, defragmentation and backup is better if partition
is small).
Basically it is a good idea to keep user data and XP
installation seperate -> means you keep user data on d:
and c: is only XP plus software ...... you need about 5-10
Gbyte depending on the amount of software you plan to
install (If you have lots of games ....you need more space
for them).
I would even recommend to keep XP installation and at
least large programms in diffreent partitions. That gives
an even smaller c: drive.