Do you want to change the plot width and height? Do you want to change the background color of the plot? All these can be done via the plot configure window. You can always bring out the plot configure window by clicking the button on the right-bottom corner:
You will then see the plot configure window:
Here you can make configuration for the current editing plot. There is an option named "Rumple", which can make your prototype looks more unfinished. You can drag the slider on the right to change the strength of the rumple effect.
Remarks: If you like to change the preferred parameters for newly created plot, you would need to do it in the settings window.