Problem solved: The slider dialog is constructed in ControlPanel.rb by:
$PhysicsControlPanelDialog=UI::WebDialog.new("SketchyPhysics Control Panel", true,"bzasdfasdfa12ctrasaa123", 290, 200, 1000,800, true)
I looked up WebDialog.new and guessed from the missing description that the parameters with values here 1000 and 800 are width and height. Like cdauscher, I'm trying to use the stuff on a notebook - replacing 1000 and 800 with e.g. 600 and 400, I get a small but nice window of sliders.
Bug in WebKit: windows that are as big or bigger than screen size (mine is 1280*800) don't get displayed.