There are many threads concerning linux screen resolutions and small 'full screen' issues. I have found the following:- When checking Linux video settings I have found it was set to VESA not the nvidia driver of the MacPro. Trying to set it to NV or nvidia failed. In the end I set the video memory to the max. 256MB and 'automatic' and reboooted and it came up fullscreen, finally. I suppose this is a limitation of the guest operating system, having to use VESA instead of the NV or Nvidia drivers. This should be spelt out as a lot of people are having linux guest screen resolution issues.
