The larger the zone, the lower the probability of sighting. I suggest using a d10 mechanism for determining sighting. You could have modifiers based on intelligence, weather, e.g. if a ship or squadron is actively looking for enemy presence in a given area because of xyz, add 2 to the dice roll; if weather is foggy, subtract 1 from the dice roll.

What is the time duration of the campaign? The more we know what you envision, the more direct our input can be.