Ubuntu – Screen Resolution Too Small

OK, it looks like I solved this problem, somehow or other. Here's what I did:

1. I quit VirtualBox (both the vm and the graphical VirtualBox Manager window);
2. At the Windows host's cmd prompt, I ran:
   
   C:\>cd "c:\Program Files\Oracle\VirtualBox"
C:\Program Files\Oracle\VirtualBox>VboxManage setextradata "<vmname>" "CustomVideoMode1" "1280x800x16"
   
   (Of course, replace <vmname> with the actual name of your vm, and replace the resolution spec string with the appropriate one for your monitor. Also note that the last character in the argument CustomVideoMode1 is the digit 1, and not the letter "l". With some fonts the two characters are practically indistinguishable. BTW, more custom models can be specified, using a similar command, with arguments of the form CustomVideoMode2, CustomVideoMode3, and so on up to, I believe, CustomVideoMode16.)
3. I restarted VirtualBox and the VM; (at this point I noted that there was no change in the appearance of the display, nor in the resolution options available through System Settings > Displays);
4. I selected System Settings > Additional Drivers, selected the option Oracle VM VirtualBox Guest Additions for Linux Module, and clicked on Activate;
5. I restarted the vm;
6. I wiped the tears of joy from my eyes.

When the VM came back up, the resolution was the correct one; furthermore, now the resolution options available through System Settings > Displays include one additional option "1280 x 800 (16:10)" (it is, of course, the currently active one).

I can't tell whether running VBoxManage setextradata ... (steps 1 & 2 above) made any difference. IOW, steps 4 & 5 may very well have done the trick.

Also, note that the Oracle VM VirtualBox Guest Additions for Linux Module was not activated even though I had previously run Devices > Install Guest Additions... at the VirtualBox control panel.