I am not sure the nature of the problem as it could be one of two things
from the way that you describe it. It could be the image size in VB (most
likely) if the message is a Windows one.* That would mean the image is a
fixed size and is not large enough. If it is an Ubuntu message then it is
possible that you opted for a dynamic image size and it is taking up too
much space in your home directory.*

If it is the former then try this:
askubuntu.com/.../how-do-i-increase-the-hard-disk-size-of-the\
-virtual-machine

If it is the latter then use the Live CD and Partition Editor or Parted
Magic to shrink another partition and enlarge home or root depending how
many Linux partitions you are using. See:
http://partedmagic.com/doku.php?id=using_gparted