Hello Benedikt,
From what I understand, you want to change icon or its properties for certain elements that are stereotyped in System Composer.
You can find the option to select custom icon from the "Profile Editor" app button present in “Profiles” section under “Modeling” tab for System Composer toolstrip. Currently, it supports selecting a “custom icon” to be used for your stereotyped element. However, the icon-size change is not available for pre-defined icons. Here is a screenshot from the examples on "TrafficLight" for you to refer.
You can further refer this MathWorks Documentation on defining and styling Stereotypes in Profiles: https://www.mathworks.com/help/systemcomposer/ug/customize-model-elements.html.
Hope this helps!
