The typical rule of thumb is at least 1.5 times the inlet, with the outlet placed at a low pressure zone. (info from NACA reports)
That slowly crept up to 2 - 3 - 4 times the inlet, whilst forgetting the low pressure outlet. Now I often see a large outlet that allows the firewall to act like an air scoop, to let air in, not out. With two openings struggling for priority to let air in, the cowl is a bad place for the engine to be in.