Depends. You can get by on very little. I'd recommend you make it 10-15 gigs in size.
You don't want the pagefile on the same drive as your OS if you can avoid it, but if it has to be on the same drive, you are better off putting it on the same partition as the os.
After the OS partition I like to make a programs partition of about 20 gigs, where I install everything to, I do it for a couple reasons, none of which are really that important, but if you don't want to do that, make your OS partition lots bigger then, because its going to have to handle every program you ever install.
After that, I recommend you make at least 1 more partition. That way if you need to reinstall your OS you don't kill all your other files like documents, music, movies, ect. You don't need a partition for each of those anymore, it was a good idea back in the day of 98 when it had to be fat32, but with NTFS you can have large partitions and just seperate the stuff into their own directories without any cluster size issues.