По умолчанию размер экрана - 640×480. Чтобы увеличить, надо лезть в…Grub

Украдено здесь: http://askubuntu.com/questions/456527/maximum-terminal-resolution-in-ubuntu-server-virtual-box-guest

The answer I found was a combination of How do I increase console-mode resolution? and How to use ubuntu server full screen in virtualbox?. The former provided information for changing grub's display resolution, and the latter assisted me in diagnosing why the former wasn't resulting in any resolution change.

Combining the two answers:

Identify the resolution that your VirtualBox environment supports. Reboot the system and hit c when the grub windows appears appears. This will drop you into the grub console. Enter the commend vbeinfo to see a list of resolutions. Select the resolution that works for you (in my case, 1152x864x32).

Edit /etc/default/grub to modify/create a line

GRUB_GFXMODE=1280x1024x32

(where you replace the resolution with the one appropriate for your environment).

Edit /etc/grub.d/00_header to modify the line

if [ "x${GRUB_GFXMODE}" = "x" ] ; then GRUB_GFXMODE=auto ; fi

to be

if [ "x${GRUB_GFXMODE}" = "x" ] ; then GRUB_GFXMODE=1280x1024x32 ; fi

(again, replacing the resolution)

Update grub and reboot:

update-grub2 && shutdown -r now

Grub should boot at the new, higher resolution, and then that resolution should continue through after you are logged in.