It depends on the theme used. You could change your theme, or edit the theme if you really wanted to, but an easier and more consistent way is to hold down Alt and drag with the middle mouse button. You can imagine it as slicing the window into a 3x3 grid, and clicking in any rectangle but the center one will allow you to drag the appropriate side or corner.
Note that Alt + MiddleButton is the default configuration. To change it (at least in Compiz):
- Go to System -> Preferences -> CompizConfig Settings Manager if running classic Gnome, or search for CompizConfig Settings Manager in the Dash if running Unity.
- Find the Resize Window plugin; it's under Window Management.
- Open it and go to the Bindings tab if you aren't already there.
The last two settings are called Initiate Window Resize. One of them is the middle click option described above; the other is a similar keyboard binding that allows you to resize with the keyboard arrow keys (as well as the mouse without clicking).