virtualbox.org

I was trying to resize it down from 42 GB to 32 GB but I goofed. Now trying with the correct size fails with
Fortunately I have spare copies of the VDI file on another machine so it's ok if I can't recover but would be nice to know how if I can.

No official tool will let you shrink disks.

Search for "vidma". It's the only tool that allows you to shrink a disk, that I know of. Use at your own risk.

Another thing you might do is boot up in a Linux version that supports huge drives, and dd the first portion of the drive to a second drive.