virtualbox
I had Ubuntu 10.4 working fine with the previous version of VirtualBox on a Windows 7 host.
Then I upgraded to VirtualBox to 3.2.10, reinstalled the Linux Guest Additions, and now I am limited to 1008 x 584 resolution in full-screen mode! I have reinstalled Guest Additions and rebooted the guest OS several times.
Install the latest version of virtualbox say 4.3.10 or above from here and also it's corresponding guest additions, extensions Or you can also install the latest virtualbox version from the multiverse repository via apt-get. I also faced the same problem in older virtualbox version running Ubuntu 14.04 as guest OS.
To install guest additions, follow this answer.
In VirtualBox, you have a menu that forces a user-specified screen resolution for the virtual screen. It should help you to specify Lubuntu the correct resolution:
Best Answer
Virtualbox 3.2.12 has been out for a while, maybe that will fix your issue