Minimum size for /home?

[postid <postid@att.net>]

I'm setting up a laptop for triple boot: WinXP; Debian Lenny with KDE
and lots of bells and whistles; Debian Lenny with Fluxbox and just the
essentials. I may swap one of those three for a different distro later.
Here's the planned layout:

hda1 -------- WinXP
hda2 -------- / for first distro
hda3 -------- / for second distro
hda4 -------- extended
  hda5 -------- /home for first distro
  hda6 -------- /home for second distro
  hda7 -------- shared data partition
  hda8 -------- swap

All data will be stored in the shared data partition, except for what's
in the dot files (e-mail, etc.) in the /home partitions. (I'm not using
a shared /home partition since I want to avoid config conflicts.) I have
limited hard-drive space.

Here are the questions:

-- How small can I make the /home partitions?

-- With data being stored in its own partition, what are the limiting
factors for /home partition size? I suspect it's e-mail attachments that 
could bloat /home the quickest.

On the other hand, I could just let /home be a part of / for each distro 
(as below) and back up the dot files regularly.

hda1 -------- WinXP
hda2 -------- / for first distro
hda3 -------- / for second distro
hda4 -------- extended
  hda5 -------- shared data partition
  hda6 -------- swap