Just wanted to point out that I have the same problem here. While the
differently sized desktops worked well with X on Squeeze they don't
after upgrading to Wheezy.
If you move a window into the "black spot" of the smaller monitor then
you have no chance to find it and draw it back. Also icons put there are
I'm working with a Lenovo X201 laptop in a docking station with a larger
external monitor. With Squeeze I could switch off the laptop's monitor
and just use the external monitor. Now if I try that both monitors go
dark and after a while the Gnome display settings dialog reappears as I
didn't confirm the new settings.
Ubuntu 11.04 is suffering from the same problem which was the reason I
went back to Debian on the laptop.