I would set the maximum size, minimum size and remove the gripper icon of the window.
Set properties (MaximumSize, MinimumSize, and SizeGripStyle):
this.MaximumSize = new System.Drawing.Size(500, 550);
this.MinimumSize = new System.Drawing.Size(500, 550);
this.SizeGripStyle = System.Windows.Forms.SizeGripStyle.Hide;