virtualbox.org

I'm running Ubuntu 14.04 as a guest on a 64-bit Windows 7 environment.

The 5.1.8 VirtualBox extension shows up as being installed. The display memory size is set to 128MB. But when I go into the Ubuntu Settings->Display, the only option is 640x480. This screen size is useless as some of the windows (like Display settings) don't completely show and aren't scrollable.

Do I need to recreate the guest virtual machine and do another Ubuntu install?

No, most probably you need to install the Guest Additions. Please take a look at the User Manual (https://www.virtualbox.org/manual/), Ch. 4.2.2 Guest Additions for Linux, or take a look at https://www.virtualbox.org/wiki/LinuxAdditionsDebug

Is this by any chance the Ubuntu Server?

In the manager, changed the Preferences -> Display -> Maximum Guest Screen Size from Auto to None
Created a new VM and installed Ubuntu 14.04.
Ran Devices -> Insert Guest Additions CD Image
This brought up a terminal session which ran some commands
After rebooting, I was able to change the Ubuntu settings for display to something useful

Something must have gone wrong in the original VM adding the guest additions. I ran the process again and it said that they were already installed but when I told it to go ahead, it reinstalled them and then the larger screen sizes could be selected.