Hello Matthias,
It would be easier to assist you with a link to your project. The most reliable way of increasing the width of a slider scale element is to use CSS rules in a CSS file named global_*.css in the project’s Aesthetics folder, as described on this thread. To overwrite the constraint on the slider’s maximum length that PennControler sets by default, add max-width: unset !important; to the CSS rules that apply to the Scale element (.PennController-MainSlider in the example above)
Jeremy