Depends on the size of your HD, but I like to set my C: partition to about 80gig since this is also where the desktop working/storage space is located, and it may get a little loaded before I move things.
Fully loaded, my C: drive runs about 13 gig full.
Then I set the remaining HD space for storage and the working temp space for some programs.
Depending on your computer and the applications you like to run - The best arrangment might be to have your C: drive on a separate HD, say an 80gig, then use your Slave for storage and temp working space.
This way, both HDs can be working at the same time .. "basically stated"