It is possible to make them smaller. Its just the collision and possible jitter issues that results in them being larger than the minimum possible so they have a smaller chance of failing. I went with 3 single axis turrets though you could go to 2. If one fails to move in time or gets bugged it might not actually give a successful detection if both have to work. I also used beam slaves to increase the range making the turrets larger 7blocks instead of 5. I had mine with full rotation available. If you limit their total available rotation to the minimum required it would make it smaller. The other issue is sometimes when they do detect they might jam up and get stuck in which case your going to have to get into it and play with the rail activators or edit/undock/dock the turrets and you will need room to do so.
If you want to make one there made of single axis turrets, Have a arm on a rail move in and out of where the turrets rotate. If the turret moves to engage the target its set for the rail will collide with it and be unable to continue. Detect that the arm isn't hitting the ends within a certain amount of time to get a detection. Then it depends on how many turrets and what you want for outputs. You want atleast two so that way the target if its directly in front of one of the turrets the other atleast should rotate but its possible if your really really unlucky that two turrets line up and that's why I had three.
I've gotten some other ideas as well to possibly compact it down so I will put it on my list of things to do just no idea when I'll get a chance to work on it.
Nice work.