It is probably too late for an answer but I came access the same question and solved it by adding the height of all visible nodes in the tree:

        private void NavTree_AfterExpand(object sender, NodeEventArgs e)
        {
            UltraTree aktUltraTree = (UltraTree)sender;
            ResizeTree(aktUltraTree);
        }

        private void NavTree_AfterCollapse(object sender, NodeEventArgs e)
        {
            UltraTree aktUltraTree = (UltraTree)sender;
            ResizeTree(aktUltraTree);
        }

        private void ResizeTree(UltraTree aktUltraTree)
        {
            if (aktUltraTree.Tag != null && aktUltraTree.Tag is UltraExplorerBarGroup)
            {
                UltraExplorerBarGroup aktUltraExplorerBarGroup = (UltraExplorerBarGroup)aktUltraTree.Tag;
                int height = CalculateTreeHeight(aktUltraTree);
                aktUltraExplorerBarGroup.Settings.ContainerHeight = height + 10;
            }
        }

        private int CalculateTreeHeight(UltraTree aktUltraTree)
        {
            int retval = 0;
            AddNodeHeights(aktUltraTree.Nodes, ref retval);
            return retval;
        }

        private void AddNodeHeights(TreeNodesCollection nodes, ref int sum)
        {
            if (nodes != null && nodes.Count > 0)
            {
                foreach (UltraTreeNode aktUltraTreeNode in nodes)
                {
                    if (aktUltraTreeNode.Parent == null || aktUltraTreeNode.Parent.Expanded == true)
                    {
                        sum += aktUltraTreeNode.Bounds.Height;
                        AddNodeHeights(aktUltraTreeNode.Nodes, ref sum);
                    }
                }
            }
        }