The Preferences window contains user-controlled options for how fv behaves and looks. Click on one of the labeled tabs to bring up one of the following sets of options...
Click on "Configure..." to modify the graph size list.
To delete one of more size(s), on the "fv: Graph Size" top panel, click the button in front of the selection (could click on more than one selection.) And then click on "Delete" to delete the selection(s).
To add a new graph size, on the "fv: Graph Size" bottom panel, input new width and height. The new size (in width x height) will show up on top of "Available Graph Size" if it is a new one.
Click "OK" or "Cancel" to exit.
Click the Save button to permanently save your preferences. Location of saved preference: