You can use the above code to change the screen size which is opened. But i don't know how useful it will be in your case. I think you can use set_param each time you use open_system.
If you used laod_system(), then the Simulink model is not supposed to be visible. open_system() will load it and make it visible. At the end of your simulation, you can run open_system() again to view it. Run the below commands line by line to see the effect.
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