When the GridListControl is initially setup for the first time it is drawn, the row heights are automaticallt set (and this steps on what you tried to do to initially set the size.)
One way around this is to subscribe to the GridListControl''s Paint event. Then in the handler code, unsubscribe to teh event so it is only hit once, and there set the default size.
private void ListBox1_Paint(object sender, PaintEventArgs e)
{
//unsubcribe to the event so it is only done once
this.ListBox1.Paint -= new PaintEventHandler(ListBox1_Paint);
this.ListBox1.Grid.Rows.DefaultSize = 50;
}