Question

Is there a way to set the default size of the window that opens up when you run the app to open up the GUI?

Answer 1

One of the constructors for the Scene object accepts two double values, a width and height for the window. If you are using FXML the line should look something like this:
scene = new Scene(loadFXML("primary"), 640, 480);

the 640 and 480 correspond to a default size of 640x480

Comments

Thank You!

Answer 2

Another way to make the screen automatically full screen is

stage.setFullScreen(true);

This will set the screen to full screen mode automatically, while false does the opposite.