Sometimes the initial size of a virtual disk is just not enough, but creating a new larger disk and copy everything over will take a much longer time, especially when you already have a full disk. The easier way is simply resize the vdi file using the VBoxManage that came with Virtual Box.
1. first shut off the machine.
cd “C:\Program Files\Oracle\VirtualBox”
the directory where virtual box is installed.
VBoxManage modifyhd “C:\Users\ziliu\VirtualBox VMs\Ubuntu1\ubuntu.vdi” -–resize 81920
Locate the .vdi file you want to resize, specify the size in MB.
Then you will need to resize the partition on the virtual machine. Here is an article about how to do it