e.DrawDefault = true;
currentWith_ = Math.Max(currentWith_, e.Node.Bounds.Right);

private void treeViewAfterExpand(object sender, TreeViewEventArgs e)
{
    int maxRight = treeView.ClientSize.Widht;

    if(e.Node.Nodes != null)
        foreach (TreeNode node in e.Node.Nodes)
        {
            maxRight = Math.Max(maxRight, node.Bounds.Right);
        }

    treeView.ClientSize = new Size(maxRight, treeView.ClientSize.Height);
}