Many components in Fiddler's UI are not "public", but because of how WinForms works, you can crawl around the window until you find them. This C# FiddlerScript makes the box at the bottom of the AutoResponder tab taller.

HackyFiddlerScriptUITweaks.cs

[ToolsAction("FixPanelHeight")]
public static void DoFix()
{
  foreach (Control c in FiddlerApplication.UI.tabsViews.Controls) {
	if (c.Name == "pageResponder") foreach (Control d in (c.Controls[0] as UserControl).Controls) 
	if (d.Name == "pnlAutoResponders") foreach (Control e in (d as Panel).Controls) {
		// FiddlerApplication.Log.LogString(e.Name);
		if (e.Name == "gbResponderEditor") {
			e.Height=e.Height+120;
			e.Top=e.Top-120;
		}	
		}
		}
}