There is 2 options: the most reliable is to create a bigger disk and copy content from the first to this one (with something like backup tool), the other one is a bit tricky, you'll have to modify the geometry of virtual disk and resize partition with a partition tool like parted on Linux.
I succeed with 2nd solution with a WinXP system disk (I was bored to use 1st solution
), I have twice the size now, but the inside size of vdi is still the older. It's work, no error at all.
To do this I have just write a little patch for vditool. I'll send it soon to dev.