– To eliminate the empty box space on the right side we would like to limit the box width with e.g. 600 px.
– To have it also for mobile users we would like to make this box mobile friendly (like the other page sections).
Can you please post this question in the CSS Customisation forums? They’ll be able to help you with either the correct inline CSS, or even to create a CSS class you can use if you want boxes like that on other pages as well.