To run the batch file when the VM
user logs in:
Drag the shortcut--the one that's currently on your desktop--(or the batch file itself) to Start - All Programs - Startup. Now when you login as that user, it will launch the batch file.
Another way to do the same thing is to save the shortcut or the batch file in %AppData%\Microsoft\Windows\Start Menu\Programs\Startup\
.
As far as getting it to run full screen, it depends a bit what you mean. You can have it launch maximized by editing your batch file like this:
start "" /max "C:\Program Files\Oracle\VirtualBox\VirtualBox.exe" --comment "VM" --startvm "12dada4d-9cfd-4aa7-8353-20b4e455b3fa"
But if VirtualBox has a truly full-screen mode (where it hides even the taskbar), you'll have to look for a command-line parameter on VirtualBox.exe. I'm not familiar with that product.