Failure to set this property, or ItemWidthDefault on the UltraWebToolbar element, may result in the toolbar expanding to fill the available space on it's right, to the edge of the browser window.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2