What's the minimum size for this?

I have 12G free space, without resizing anything else yet. Ubuntu 80G and XPPro 15G are on the disk now.

AND, instead of taking swap out of the free 12G, can I use Ubuntu's swap partition for FC8 swap?

EDIT:
I did 2G for root partition.

Swap will share - got that one answered...

Well the minimum size depends on how many packages you are going to install.
The minimal install (without GUI) would be less than a gig, and the full installation (all packages)
will take about 4 to5 gig.

I think your 12 gig space is more than enough if you are not going to install a lot of packages after the install.

2 GB for the root partition may not be enough after some time.Better go with 4 or 5 GB.It all depends on what you do and what will you install eventually.Log files are getting bigger etc.

Thanks - I took it up to 5G, and found out that swap will share - now, if I can update to FC10 direct from a server, and not burn a disk, I'll be happy!

Support for Fedora 8 ended on January 7,2009,meaning there are no more updates available for it.Upgrading from Fedora 8 to Fedora 10 is not possible in the usual way,maybe you can upgrade but it could be an unpleasant expirience.So it's better if you download and install F10 and if you want to upgrade in the future do it from F10 to F11 and so on,not from F10 to F12.It's good if you have separate /home partition then you can install newer versions of the distro and just point to the /home partition during the installation,that way you'll have all your favorite things and a new system.

If disk space becomes tight you can still move /usr or /home to some other partition and make some softlinks point to those locations. Obviously you will have to change /etc/fstab to mount those partitions when booting.

As a matter of fact, you could even share your home directory between Ubuntu and Fedora this way (should work for most applications, though some might rewrite their settings each time the distro is switched).

Debian