This FiddlerScript autosizes the width of the Web Sessions list based on whether it contains focus. Adjust width constants to taste.

AutoSizeSessionList.js

// Click Rules > Customize Rules
// Inside your existing onboot handler, add two lines:
  static function OnBoot() {
		FiddlerApplication.UI.pnlSessions.add_Enter(panelEnter);
		FiddlerApplication.UI.pnlSessions.add_Leave(panelExit);
		
		
// Just before that method in the Handlers class, add:
	
	public static 
		RulesOption("AutoSizeUI")
		BindPref("fiddlerscript.AutoSizeUI")
	var m_AutoSizeUI: boolean;
		
	static function panelEnter(o: Object, ea: EventArgs)
	{
		if (!m_AutoSizeUI) return;
		if ((FiddlerApplication.UI.pnlSessions.Width <= 700) &&
			(FiddlerApplication.UI.Width > 720))
		{
			FiddlerApplication.UI.pnlSessions.Width = 700;
		}
	}
		
	static function panelExit(o: Object, ea: EventArgs)
	{
		if (!m_AutoSizeUI) return;
		
		if (FiddlerApplication.UI.pnlSessions.Width > 650)
		{
			FiddlerApplication.UI.pnlSessions.Width = 60;
		}
	}