I second the feature request to be able to extend the size of a VDI file, I would prefer to be able to have a source VDI which I clone to make new machines that is only as large as it needs to be.
In the absence of this functionality I am now using the following steps to create new Windows XP machines (I am sure these would transfer to other OS's as you are only limited by filesystem support in GPARTED):
First Obtain ISO image of EASEUS Disk Copy from
http://www.easeus.com/disk-copy/index.htm
Also you must Obtain ISO image of GPARTED Live CD from
http://gparted.sourceforge.net/livecd.php
To clone from your smaller VDI to your new larger one:
* Attach your existing (source) VDI to primary slave
* Attach your new (target) VDI to primary master
* Attach the Disk Copy ISO to the CDROM
* Ensure CDROM is selected as a boot option above Hard Drive and start VM
* Accept default boot option
* Follow on-screen prompts once Disk Copy has started:
-- "Welcome to the Disk Copy wizard" - press Next
-- "Select copy mode" - select "Disk Copy" and press Next
-- "Select source disk" - select "Disk 1" (check sizes) and press Next
-- "Select destination disk" - select "Disk 0" (check sizes) and press Next
-- "Task preview" - Check settings and press "Proceed" when happy
-- Wait for process to finish, the press Quit to exit.
-- Host+Q and powerdown machine when completed.
* Detach your source disk from Primary Slave
To expand your cloned data to fill the new VDI:
* Attach the GParted-Live ISO to the CDROM and start VM
* Accept default boot option
* Follow startup prompts to select your preferred keyboard layout, select "vesa" graphics and select your preferred language
* Once GParted has started, select your new partition and choose "Resize/Move" from toolbar
* Stretch partition to fill drive and click "Resize/Move"
* Click "Apply" on toolbar and follow prompts
* Exit GParted and Host-Q and power off VM
* Unmount CD and ensure Hard Drive is selected as a boot option
* Start VM - all done
Hope this helps some of you, it works reliably for me and involves minimal downloads and disk space for ISOs - I had previously been using UBCD as it includes all the tools required, but a larger ISO image.
Jim