The size of your screen in the virtualbox is actually the problem of the guest OS not of virtualbox itself. You'll want to change the aspect ratio in the guest OS. In Ubuntu this can be done by clicking the downward facing arrow in the top right corner of the screen, this will open a small window in that area, click on the settings button, then go down to devices, and then display, and select a display size. The correct choice will depend on your monitor, but I selected "1440X900(16:10)" and that got rid of the gray borders on my computer.