First be sure you have installed the "VirtualBox Guest Additions". This is done when you install Windows into the VM and here is that very easy FAQ to follow below in case you missed it.
FAQ How to load your Microsoft Windows licensed disk into your virtual machine with a DVD
Now sinply load Windows and click on "View" at the top left of VirtualBox and you will be able to
adjust your Windows Virtual Machine Window size. You can even "Switch to Fullscreeen" mode. Please be sure you read and remember the keystroke combination to do this, which is your Host key i.e. Your "Right Control Key" and the F key on your keyboard so that you can switch it back when you want to.
Please note you can also drag the bottom right edge to make your Windows VM larger
Click here to go to the Robolinux FAQ webpage For Ubuntu