You can switch your VNC session screen geometry or resolution on the fly by listing all the possible geometries of the screens you want to use in future while creating vncserver.

The syntax for declaring two geometries 1280×768 and 1920×1080 is

sam@sun:~$ vncserver -depth 24 -geometry 1280x768 -geometry 1920x1080

You will get the following message in the terminal

New 'sun:3 (sam)' desktop is sun:3
 
Starting applications specified in /home/sam/.vnc/xstartup
Log file is /home/sam/.vnc/sun:3.log

sun is my computer name and sam is username

Now open the vncviewer, by using the command

sam@sun:~$ vncviewer sun:3

To find the possible screen display options, run xrandr in the command terminal

sam@sun:~$ xrandr
SZ:    Pixels          Physical       Refresh
*0   1280 x 768    ( 325mm x 195mm )  *60
 1   1920 x 1080   ( 488mm x 274mm )   60
Current rotation - normal
Current reflection - none
Rotations possible - normal
Reflections possible - none

The message indicates two possible display options and the current geometry is marked with ‘*’.

To change screen setting to one that is listed above, type in the command terminal of vnc session,

sam@sun:~$ xrandr -s 1920x1000

Now you can see the change in screen geometry to 1920×1080.