If you are using the MATLAB mlrobot object, this uses a hidden Simulink model that contains all of the environment details. To open the model just use the following command in MATLAB or you can also navigate to the Robotics Playground example models.
Then if you double-click in the environment block you will some environment options you can change including the arena size (see picture attached). Make sure to save the model.
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MathWorks Student Competitions Team