For new users, personal Debian boxes, home systems, and other single-user setups, a single / partition (plus swap) is probably the easiest, simplest way to go. The recommended partition type is ext4.

Для многопользовательских систем или систем с большим дисковым пространством будет лучше создать для /var, /tmp и /home собственные разделы, отдельные от /.

You might need a separate /usr/local partition if you plan to install many programs that are not part of the Debian distribution. If your machine will be a mail server, you might need to make /var/mail a separate partition. If you are setting up a server with lots of user accounts, it's generally good to have a separate, large /home partition. In general, the partitioning situation varies from computer to computer depending on its uses.

Для очень сложных систем вы должны прочитать Multi Disk HOWTO. В нём содержится подробная информация, которая будет наиболее интересна провайдерам Интернет и людям, настраивающим серверы.

With respect to the issue of swap partition size, there are many views. One rule of thumb which works well is to use as much swap as you have system memory. It also shouldn't be smaller than 512MB, in most cases. Of course, there are exceptions to these rules.

As an example, an older home machine might have 512MB of RAM and a 20GB SATA drive on /dev/sda. There might be a 8GB partition for another operating system on /dev/sda1, a 512MB swap partition on /dev/sda3 and about 11.4GB on /dev/sda2 as the Linux partition.

Про то, как будет использовано дисковое пространство можно определить по задачам, которые вы захотите установить после установки системы; смотрите Раздел D.2, «Дисковое пространство, необходимое для задач».