$ vncserver -geometry 1024x768
However, after I connect to the vnc server, the resolution is not 1024×768. I change the geometry to other resolution and the actual resolution is not changed.
So the solution is to set the Gnome 3 resolution inside the session in VNC in the Display settings of gnome. You can start it quickly by
$ gnome-control-center display
or find it out in the “Settings” for Gnome 3.