ubiquity displays only 2.6GB needed for install

Where are you getting 3GB from? Running du -bchs on /rofs (the read-only copy of the live filesystem contents) reports 1.8 GB.

This is a gparted screenshot of my last trial install, and I've not added anything or even run updates. I just installed, made sure it booted and that's all.

Hmmm, df -H shows:

lance@lance-desktop:~$ df -H
Filesystem Size Used Avail Use% Mounted on
/dev/sdb1 77G 2.7G 71G 4% /
none 1.1G 730k 1.1G 1% /dev
none 1.1G 1.3M 1.1G 1% /dev/shm
none 1.1G 103k 1.1G 1% /var/run
none 1.1G 0 1.1G 0% /var/lock

I'd have sworn earlier that previous installs were coming in about 3.06 GB according to gparted, but it's typically not something I pay incredibly close attention to.

I generally recommend 10GB minimum for / if no separate /home is being created just as a matter of practicality.

I tried recreating this on a 2.9 GB disk in a virtual machine and ran into an issue where ubiquity told me my root partition was too small (2362 MB) as 533 MB was used by a swap partition.

So I think the 2.6 GB needed can be misleading in that if the partitioner allocates some space to a swap partition then you really do need more than 2.6 GB.

A / partition smaller than 10GB is just stupid.

Well, even this soon following my last iso-testing install (same iso/md5sum as the official Beta1) my / partition is 3.10GiB. And I've so far only installed updates and tweaked my resolution using Xrandr via /etc/gdm/PreSession/Default.

So 2.6GB is simply incorrect just based on root partition size alone. No doubt everyone will want to install some additional packages, save at least a bit of data, not have their web browser crash because it can't store history, etc.

I personally think anything less than 6GB for / is a disaster looking for a place to happen. Then we should take into consideration swap.

While most newer machines will seldom use swap, we need to take into consideration the ability of laptops and netbooks to suspend and/or hibernate. My swap formula amounts to:

up to 1GB RAM > double that for SWAP
1GB to 2GB of RAM > 2GB + SWAP
over 2GB RAM > SWAP must be at least the same size as RAM to allow for suspend/hibernate

So what is realistic? Here's what the community documentation says:

https://help.ubuntu.com/community/Installation/SystemRequirements

hi gays
how I can change the size and the partition of swap
and if it 0 what happen to my Linux

I've increased the minimum size to double the size of the install, or roughly 5.2 GB. This is just above the minimum size set by the official documentation:
https://help.ubuntu.com/10.04/installation-guide/i386/minimum-hardware-reqts.html

Note that this is a hard minimum. While installs below this size are technical possible, they are not allowed by the UI.

This bug was fixed in the package ubiquity - 2.6.0

---------------
ubiquity (2.6.0) natty; urgency=low

  [ Evan Dandrea ]
  * Provide a better description for the upgrade/reinstall option that
    intends to make the difference between it and a traditional Ubuntu
    upgrade clear (LP: #752372).
  * Don't show the biggest_free option if we can't fit Ubuntu in the
    free space (LP: #751145).
  * Try calling unlock_environment a bit earlier to work around LP: 657086.
  * Be sure to back up in the dbfilter when backing up from the advanced
    partitioning page (LP: #744938).
  * Increase the minimum size of the Ubuntu install to be twice the size
    of the contents of the installation (roughly 5.2 GB) (LP: #745148).

  [ Colin Watson ]
  * Switch to xz compression for the source package, saving nearly a
    megabyte.
  * Update translations from Launchpad.
  * Update imported translations from gtk+2.0 2.24.4-0ubuntu1.
  * Automatic update of included source packages: apt-setup 1:0.49ubuntu4,
    netcfg 1.60ubuntu3.

  [ Jonathan Riddell ]
  * Update artwork gui/qt/images/squares.png
 -- Colin Watson <email address hidden> Sun, 10 Apr 2011 01:47:19 +0100

I hope you guys are aware of bug #775124 - after changing the minimum space requirement, everyone who uses Ubuntu on a 4 gb SSD can no longer install it. As someone who's done that for the past year or two, successfully, I can say that it's a shame that the UI prevents this now. Sure, it's a tight squeeze, but some people - including me - want to be able to do this.

https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/775124