Hey Reiner,
the slider in the “gallery” project format automatically stretches the slider to the width of the page; there are two possible ways to take at this point: one is adding a sidebar to this page, thus reducing the width available to the slider; and two is replacing the current slider with a shortcode slider (NIVO_SLIDER shortcode, please see theme manual Appendix 2 for more information on the parameters), inserting it into a standard-format project. Do you think one of these would match your needs?
Sincerely,
Andrii / Satori Studio