How to increase the hard disk size of the virtual machine

Open a terminal and navigate to the folder with the VirtualBox disk image, then use the following command:

VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB 

replacing YOUR_HARD_DISK and SIZE_IN_MB with your image name and desired size.

My guest OS is Ubuntu. To manage it's partitions I used GParted Live CD.

Access windows-host shared folders from ubuntu-guest

Mount the shared folder like this:

sudo mount -t vboxsf folder-name /media/windows-shared

See also this post.