How to recover from virtual screen miles bigger than the real screen?

[Chris G]

I just changed the resolution to 1600x1200 using xubuntu Settings
Manager for another user login on my desktop machine and I've ended up
with (I think) a huge virtual screen on which I can't see or do
anything. It's not that the display can't cope, it's working happily,
but there's nothing except background visible on screen.

How do I sort this out, get control back again.  All I can do at the
moment is switch to another virtual login.  If I log in again as this
user I just get the blank background again.

I already have the resolution set to 1600x1200 for my normal user
login and there it works perfectly.

Also, how do I get this user swithched to 1600x1200 properly?  All I
want is for it to work the same as for the other user that works fine
at 1600x1200.

-- 
Chris Green