Code

Box currentBox;
TextBox textBox;

private void fc_EnterInplaceEditMode(object sender, MindFusion.Diagramming.WinForms.InplaceEditArgs e)
{
   currentBox = e.Node as Box;
   textBox = e.TextBox;
   textBox.SelectionStart = e.TextBox.Text.Length;
   textBox.TextChanged += new EventHandler(TextBox_TextChanged);
}

private void TextBox_TextChanged(object sender, EventArgs e)
{
   Box temp = new Box(fc);
   RectangleF rect = currentBox.BoundingRect;
   rect.Inflate(10, 10);
   temp.BoundingRect = rect;
   temp.Text = textBox.Text;

   temp.FitSizeToText(FitSize.KeepRatio);
   currentBox.BoundingRect = temp.BoundingRect;

   Rectangle clientRect = fc.DocToClient(temp.BoundingRect);
   textBox.Location = clientRect.Location;
   textBox.Size = clientRect.Size;
}