First, right-click on your graph and unselect "advanced...auto adjust scales". This will prevent automatic resizing of the graph area as e.g. the axes need more room.
Now create a property node for "Plot Area...Size". You can use it to either read the current size or to set the size to your desired dimensions.
Also, if you manually change the size of the plot area (e.g. by dragging one corner), there will be a tip strip indicating the current size.