I am using old Photoshop CS6 on 21" iMac (OS 10.8.5) with 30" Cinema display. When I take only my iMac on a trip, if I had Photoshop sized for the 30" display, the bottom right of the workspace runs outside the 21" active monitor. How can I resize it to fit without having the 30" to do it?
Long time since I've seen Mountain Lion or CS6... I'm presuming you can't see the red/yellow/green dots at top left of the window?
If you can, the green dot will resize it.
If you can't, Opt ⌥ Shift ⇧ & drag any visible window edge will resize the entire window equally [I hope that functionality existed at 10.8]. Once you can see all 4 sides, you can then drag them out to the correct edges again individually if needed.
If that doesn't work, see if the window will just drag down the screen - I've known the title bar with the dots to hide itself under the menu bar, which most Mac Apps can't do.