virtualbox.org

The virtual monitor appears to be fixed at resolution 800 x 600.

When I enlarge the host window holding the virtual monitor and select Machine > Adjust Window Size, the host window shrinks to hold the 800 x 600 virtual monitor.

Also, if I select Machine > Enter Fullscreen Mode, the virtual monitor fills up a small portion of the real monitor.

I'm guessing I'm missing something, but what?

Install the guest additions (in the guest)