Hi @Newbie_1 ,
I know you can set the default size for the height: File > Preferences > Graphs. In the panel "Graphs", you can set the graph height in pixels. I don't see how to set the width, though.
However, you can always script it up. If you reference your graph as gb or something, you can send it the command gb << Set Graph Size (x, y), where x and y are the pixel sizes of the graph. See for example the Scripting Index help on Graph Size.
Hope this helps!,
DS