Windows Server 2012 default desktop size

I am using Windows Server 2012 r2 Datacentre edition running on an Azure virtual machine and I have come across a strange behaviour that hopefully someone can help me understand.

Put simply, when I use AutoLogon to ensure that a desktop exists for a particular user after restart, the desktop size appears to always be 1024×768 – I have attempted to change this using a multitude of routes including code but whichever way I change the resolution the method/api call/powershell cmdlet always appears to succeed but then on subsequent queries it's back to 1024×768. I first though that this was some default imposed by Windows when there is no display driver but after writing an app that enumerates supported display modes it returns a lot more supported resolutions. What's more bizarre is that if I use a remote desktop tool and set the resolution to something like 1920×1080 then the desktop is resized and even after the remote session is finished, queries for the desktop size now return the new resolution of 1920×1080… until the VM restarts.

I have spent a while now researching this phenomena to no avail – manually changing every instance of 1024 and 768 in the registry doesn't no anything either.

Does anyone have any idea how I would change the default desktop size for a user when there is no display device and where this 1024×768 "magic" resolution comes from?

EDIT: Thanks for the answers so far and although I appreciate them I am already pretty certain that this isn't going to be resolved with a simple reg hack or startup tool (mainly because I've already tried them) – I am pretty sure the problems lies in the fact that there is a display driver (Hyper-V Video) but no display device

Thanks