resolution problem by using Ubuntu in virtualbox

I am using a macbook pro, where I installed a virtualbox in order to use some virtual machines.

when a Ubuntu machine is installed, the maximum resolution of the screen is 800*600, which is not enough for a big screen.

so, to solve this problem, you need only to install

virtualbox-ose-guest-x11

by using either apt-get install or Synaptic Package Manager.