I just installed Ubuntu 9.04 and AFAIK this bug was no longer present.
** Changed in: ubiquity (Ubuntu)
Status: New = Fix Released
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disk usage bar in manual partitioner not displaying partition sizes correctly
https://bugs.launchpad.net/bugs/295882
You received this bug notification
** Attachment added: screenshot of Ubiquity's manual partitioner with
partitions set up for installation
http://launchpadlibrarian.net/19506400/ubiquity_manualparted.png
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disk usage bar in manual partitioner not displaying partition sizes correctly
https://bugs.launchpad.net/bugs/295882