BetterYTM-Mirror/assets/style/volSliderSize.css

/* {WIDTH} will be replaced with the width number set in the config menu */
#bytm-vol-slider-cont tp-yt-paper-slider#volume-slider {
  --bytm-slider-width: /*{WIDTH}*/;
  width: var(--bytm-slider-width, 120px) !important;
}