On my installation of Ubuntu 14.04 in VirtualBox, the only screen resolution available is 640 x 480. I have tried the solutions suggested in this answer, but none of them helped. All of the answers there are about installing the Guest additions, virtualbox-guest-dkms, but that doesn't make any difference on my system (Win7).
After having first installed the Guest additions, which didn't work, I found a solution elsewhere: linuxbsdos.com.
Use Xdiagnose from the Dashboard. Search for and launch Xdiagnose, then enable all the options under the Debug section. Click the Apply button, then close the window and restart the system.
That's what finally did the trick! Now I get 1024 x 768 instead.