In order to get the full resolution on a virtual machine you'll need to install Virtual PC Additions.
To do this you'll need to do the following
While you are running one of your virtual machines, click on Action
Then click on Install or Update Virtual Machine Additions (or hit Right Alt + I)
3 . You will be prompted with the window above which states that a CD image will be inserted in the Virtual Drive, the CD-ROM contains the installation files for the Virtual Machine Additions. Click Continue so that the virtual CD-ROM is loaded.
From there just continue installation and you'll be able to take advantage of the full screen.