Maybe it's 'Maximize windows by dragging them to the top of the screen'?
You can find it under 'Workspace Appearance and Behavior' -> 'Workspace Behavior' -> 'Screen Edges' -> 'Window Management'.
My setup is 3 monitors side by side and windowsizes do not change here when dragging from one to an other...
Maximize and tile windows are both checked on my system. (fglrx, monitors set up with xrandr)