I am running Windows 7 as a guest on a Windows 8.1 computer. I wanted my virtual Windows 7 computer to have a screen resolution of 1280x800. This is to emulate a small screen for some training. I found that in the Hyper-V window I could not change the screen resolution to 1280x800. I right clicked the desktop, clicked Screen Resolution and Resolution, others were available but not the resolution I wanted, 1280x800.
Using Remote Desktop Connection (mstsc) connect to the virtual 'guest' computer from the host. In the Remote Desktop Connection settings you can change the resolution to whatever you like.
Press the Windows Key and R to get the Run dialogue box
Type mstsc and click OK
Click the Display tab and change the Display configuration as shown above.
Using the Remote Desktop Connection to connect to your virtual guest computer is fine. In my case my virtual computer is stored on my C: drive but this doesn't matter, as long as there is a network connection to it (you have to have a network card configured on your virtual guest PC) then it works like a charm).
An added advantage is that you can easily copy/paste to/from the Remote Desktop Connection screen. Screen capture for example, it's much easier. For example, I used Greenshot to take a screen shot in the Remote Desktop Connection window and paste it directly into an e-mail in Outlook on my host PC.
I found some help here: