Resize a VirtualBox hard disk (.VDI) in Linux

I would recommend backing up your current vdi file before doing this!

Run this command in terminal:

VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB

Example:

VBoxManage modifyhd MyWindows7.vdi --resize 20000

It resizes your hard disk to 20GB. Start your guest machine and resize the partition with your suitable tool. (for example: Gparted for Linux or Windows Partition Manager for windows)