32Mb, holds the kernel and boot maps etc. It might be a bit large for the average system, but since disks are large enough these days, you'd better be save then sorry. If you are short on disk space, one could get around with about 10Mb.
This directory contains everything that will be installed by you from source. So it is up to you to decide how much room you want to use for this. I'll normally use 500 Mb, just to be sure again.
This is dependend on the software you are going to use. As an average use 500 Mb as start, and the add:
+1GB
+2GB
Normally on an Internet server no users should be available so this directory holds nothing at all. However if you want users or yourself to be able to login, make it a separate partition. Or if you use the Mailbox format it might be wise to use a separate partition for /home and use quota's.
1 GB should be enough. But do the above calculations first and just assign the rest of your disk to /, or if you like to play it save and don't have any other means of backup. Devide the rest of the diskspace by 2, use one half for / and the other half to make backups.