Mmmkay, sorry to hang you dry so long. This is how I did it. It's not a resize in the sense that you stretch the original file, but the end result is basically the same. I've done this with WinXP installs.
1. Duplicate the .hdd file of the VM (I'll call the original #1 and the duplicate #2).
2. Create a new empty .hdd file (#3) of the desired size.
3. Map #3 as 0:0 (Primary Master).
4. Install the OS.
5. Map the #1 to 0:0, #2 to 1:0, and #3 to 1:1.
6. Once booted, remove everything except for boot-related files on #3, and copy over everything from #2 to #3.
7. Map #3 as 0:0 again and if the booting is successful, discard #1 and #2.
Now, XP and Vista have significantly different boot-related files. This Wikipedia article might be of some help in identifying what to preserve/remove in case of Vista:
In case of XP, I preserved NTLDR, NTDETECT.COM, and BOOT.INI.
When in doubt, you could make a backup of #3 after stage 4 and play around with the boot-related files to see what happens.
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