Increase size:
1. vboxmanage createhd –filename <name.vdi> –size <dest bigger than src in mb> –format VDI –variant Fixed|Standard
2. vboxmanage clonehd <src.vdi> <dest.vdi> –existing

Decrease size:
1. Defragment your hard drive in Windows
2. Use sdelete (1.61) to fill empty space with 0s in Windows (sdelete -z c:) [DOWNLOAD]
3. vboxmanage createhd –filename <name.vdi> –size <dest as big as src in mb> –format VDI
4. vboxmanage clonehd <src.vdi> <dest.vdi> –existing
5. Run Gparted Live ISO in VBox to shrink your (re)partition
6. Now you’ve got a dynamic VDI with fixed disk partition size and the possibility to increase the space via Gparted as you like

Bind your cloned VDI to your VBox