I am using Virtual Box 5.0.0(Windows Vista) to run Ubuntu 14.04. The screen is too small (640x480) but the screen can up to 1280x768. How can I resizing it?
You need to install the VirtualBox Guest Additions into Ubuntu. This gives a few benefits, including resizing the Ubuntu screen to match the host window.
Start the VM, then select from the VirtualBox menus:
CD/DVD Devices ->
VBoxGuestAdditions<version>.iso. Then follow the instructions.