The Slackware book has some good info about what to use partitions for, but the simplest rule is to have 3 partitions
/,
/home &
swap.
Swap should be about twice the size of your RAM, thought there are some who say it shouldn't be more than 1Gb.
/ should be at least 3Gb, and depending on what apps your are going to install, then 5Gb should be OK if it isn't your main OS. If your going to use it for testing then a separate
/home means you can keep your data if you have to re-install or upgrade. With multiple boots, always have a
/home for each, as that's where all the application user data goes, and if you are playing with one you don't want it to mess up the other. If Slackware is only for playing, the
/home partition doesn't have to be big.
If you want to learn about partitioning, a good place to start is the
GParted website. Download the LiveCD iso and burn that. It will give you a rescue boot disk with a useful GUI and editing tools, plus a very good partitioning tool. I have used it to resize NTFS partitions for Windows XP, as well as for all my Linux partitioning.
Slackware is a good choice for learning how Linux works. Good luck.
