You can add a style tag in the ui.r tag:
tags$head( tags$style(type="text/css", ".jslider { max-width: 200px; }") ),
Put these lines inside shinyUI (), and you can change the width of the sliderinput.
UPDATE:
From Shiny 0.11, RStudio switched from jslider to irs ( http://cran.r-project.org/web/packages/shiny/NEWS ):
Changed the sliders from jquery-slider to ion.rangeSlider. These sliders have an improved appearance, support the update of additional properties from the server, and it can be controlled using keyboard input. What does it mean .jslider will no longer work.
Replace with .irs {max-width: 200px;} should work.
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