Hi: I played for years with multi boot and I came up with the following:
1. The first partition on a hd I always put a FAT (2GB or so) In case of an emergency I can boot with
a MS-DOS diskette and do some troubleshooting with DOS utils. Althougt FAT 32 is not safe.
2. Then I set up a /boot partition which I later mount as /boot - also with other Linux distros. 1-2 GB is enough.
3. The /swap partition, as a rule of thump should be 2 x RAM. So f.i. if you have 500 MB of RAM, make it 1 GB.
4. Than you make your / root partition for the first Linus distro, here you you use 5GB-10GB, depens on the work you want to do. Burning large iso files needs a lot of space. So you can do the math here.
5. Then you can make several more partitions for later use and leafe them empty. At the end of the hd I always make a 2-3GB FAT partition, which I use as an area of "exchange". Here I can exchange data between NTFS, ext3 and FAT 32.
I like the boot loader grub, but have not played around with the new grub2. I prefer to put the grub into the master boot record, once I install the first Linux distrio on my hd. All subsequent distros I install without a bootloader. Then I adjust the menu,lst according to the new distribution. Because all subsequent distris need a partition to boot from, I select the respective /root partition or some times the /boot partition. In the later case you must make sure that the kernel files etc. have different names and that the lines in menu.lst point to the respecting name.
I this will help you.