Code

diagramView.ShowToolTips = true;
diagramView.ToolTipControl.OwnerDraw = true;
diagramView.ToolTipControl.Draw += (s, args) =>
{
	var font = new Font("Courier New", 20);
	args.Graphics.FillRectangle(
		Brushes.White, args.Bounds);
	args.Graphics.DrawString(
		args.ToolTipText, font, Brushes.Black, 0, 0);
	font.Dispose();
};
diagramView.ToolTipControl.Popup += (s, args) =>
{
	var font = new Font("Courier New", 20);
	args.ToolTipSize = TextRenderer.MeasureText(
		diagramView.ToolTipControl.GetToolTip(diagramView), font);
	font.Dispose();
};