virtualbox.org

I was this thread:
posting.php?mode=reply&f=7&t=74084

But it did not solve my problem....

issue#1:
I also have the same problem as the above post.
The VM window area only covers about 50% of the maximized Virtual Box Window area.
However, my host is Linux Mint 17.1, and I
want to virtualize CentOS.
The CentOS is also graphical (gnome).

Currently I have :
virtualbox-5.0_5.0.16-105871~Ubuntu~trusty_amd64.deb

Can I enlarge the VM display area to occupy the entire area of the Virtual box window?
I tried scaling the display, but that only magnify the fonts, and not maximize the screen area to fill the entire area of VB window.

I also notice when I install a Linux mint VM, its display area cover the entire window area of VB window and
thus works like a normal screen. So why it is not working for CentOS as well as Ubuntu!

Issue #2:
I have seen these package installation :
sudo apt-get install virtualbox-guest-X11
sudo apt-get install virtualbox-dkms

What do the packages do?
Do they install VB?

Thank you.