From here.

First, resize the VDI disk file.

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# Using Linux as a Host:
VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB

# Using Mac as a Host:
/Applications/VirtualBox.app/Contents/MacOS/VBoxManage modifyhd YOUR_HARD_DISK.vdi –resize SIZE_IN_MB

# Using Windows as a Host:
C:\Program Files\Oracle\VirtualBox\VboxManage.exe modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB

Second, expend the partition of your VM with the increased disk space.

In Ubuntu, this can be done by “Disk Utility”. In Windows 7, this can be done by “Contol Panel -> Administrative Tools -> Computer Management -> Disk Management”.

If you can’t expend the partition where the OS is, please use GParted live CD to do so.