In order to get a tighter integration with the host OS you need to install the VirtualBox GuestAdditions in the VirtualBox guest OS.
This allows you to stretch the virtual computer window to an arbitrary size and have the contents scale accordingly, and it allows you to run it in a true full-screen mode. In full-screen mode the guest virtual computer takes over all of the screen. The edges of the window in which it is running are removed so that the display is exactly as if you were sitting at a (for example) Windows XP computer, instead of (for example) an Ubuntu one. You can even run the guest OS in a seamless mode, where the guest applications appear to run on the host desktop.
Installing the GuestAdditions also allows integration to shared folders on the Host OS.
To install the GuestAdditions select the Install GuestAdditions option from the Devices menu in the guest OS.