By installing a Ubuntu guest with VirtualBox on Windows, you likely found a basic resolution, such as 1024*768.
Unfortunately, you have a screen with a 1920*1200 resolution, it's impossible to let this like that.
There is of course a solution to increase this basic setup, but with a maximum of 1440*1050.
Better than nothing.
Let's do it.
Open a Terminal and write it down this line:
sudo apt-get install virtualbox-guest-x11
Restart your Linux, and you're done.
This kind of library should have been installed by default.
I think it will in the future.
Anyway, good job, you are now ready to code.