I have a iMac running 10.9.5 and have two users.
When I log in, the desktop picture and dock fills the screen as it should, but on the first switch to the second user, both desktop picture and dock are not filling the screen, but centred on the top left corner about a 15" screen size.
The dock does revert to the full size after a few seconds, but the desktop picture does not.
If I switch back to the first user and then back to the second, the picture fills the screen, as it does if I go into preferences and manually change the resolution.
I have asked another forum about this with no success. I have also repaired permissions several times without result.