After looking at it with
Firebug, it appears that the width in the widget settings is being overridden by a width of 100% defined in CSS.
You can remove the override by looking for the following code in the css file for the widget ({plugin-folder}/s5_tabshow/styles.css) around line 146:
#s5_button_wrap0, #s5_tabshow_left, #s5_button, #s5_tabshow_right {
width: 100% !important;
}
If you either remove or comment out the line for the width, it will start using the width defined in the settings.