@gimli Welcome to the forum. For those who don"t know, Gimli is from XBMC and has provided the vast majority of the work in making XBMC on R-Pi happen.
@nimdy possibly it is a bad value in guisettings.xml, so try deleting that (it will be recreated).
It"s possible XBMC doesn"t play well when forcing a non-preferred HDMI mode. If you still have a problem after deleting guisettings.xml then post your EDID dump (with tvservice) and the xbmc.log and I"ll look into it.
I've had a play around with deleting the guisettings file, but it still comes back up the same, in about an 8th of the screen. The calibration will let me make it bigger, but only to a quarter of the screen (the resizing keeps going, but doesnt fill the screen).
I did an edid dump a while back, I hope its still valid:
And here is the xbmc log with a fresh card with no config.txt or guisettings file:
I would love to get this working. So far i haven't been able to get anything working properly except the Debain image and wireless.