New Post: problem for set LayoutAnchorable floatingWidth

I have a problem with setting width for layoutAnchorable that show as float by code below :

var rightBar = DockingManager.RightSidePanel.Children[0].Children[0].Model as LayoutAnchorable;

        if (rightBar == null)
            return;

        rightBar.FloatingHeight = 400;
        rightBar.FloatingWidth = 50;
        rightBar.FloatingTop = 200;
        rightBar.FloatingLeft = Screen.PrimaryScreen.WorkingArea.Right - 170;
        rightBar.Float();

why i can not set width for it less than some value ?