This can be a hard question to answer as there are numerous factors that need to be considered. However, I will give you my take on what is the best length of hair rigs for catching carp.
Generally with larger boilies I would use longer hairs. The width of the bait means the hook requires more space in order for it to fall loosely and turn into the flesh of the mouth. With smaller baits I would use shorter hair rigs as there’s less need for space for the hook to turn properly.
I would definitely use short hairs if using a large hook with a small bait. There are not many situations where I would do this, except if the large carp in a lake prefer small baits. I could then use small food items in a PVA bag and hide a bigger hook under the mass of bait. A big hook has more chance of catching a hold in the flesh of a big carp’s mouth, and this is the reason why I would use this angling tactic. I have even used a big hook without a bait and placed the rig inside small PVA bags of pellets and caught a few carp. This is especially effective if carp are sucking at whole patches of bait. This way you don’t need to worry about the length of the hair at all!!
I think short hairs can cause problems for the hook setting properly once in a carp’s mouth. The closeness of the bait to the hook may negatively affect the ability of the hook to turn and penetrate the flesh, especially if there’s less freedom for the bait to move away or down the shank of the hook. Insecure hook penetrations caused from having too short a hair rig with bottom boilies, could result in many more hook pulls and the loss of numerous hooked fish. The weight of the boilie can flip the hook over the wrong way where the point penetrates the flesh loosely.
A short hair may cause the hook bait to act abnormal as a strong current or feeding fish move it about on the bottom. The weight of the hook and the lack of freedom make it behave unnaturally because the hook weight limits its movement when lying on the bottom. This abnormal movement may make carp suspicious of the bait and avoid it altogether.
On the positive side, a short hair rig may be best if the fish in the lake tend to feed cautiously. As a carp quickly sucks in and blows back food items, the hook should have a greater chance to catch in the side of the lips for hooking. With longer hairs the bait may not get chance to enter the mouth altogether if carp suck and blow quickly!
I also prefer to use short hairs for pop-up boilies. I like the hair to come off the hook about halfway down the shank. This allows the hook to kink over in a claw-like position, which I feel produces great hooking potential.
I find longer hairs produce more takes, especially with a longer fishing rig and a bigger boilie. I have known some good anglers using hair rigs of up to 2 inches long!
In the past, some anglers used to tie a long hair rig from the bend of the hook to the boilie, using lengths of up to 2 inches. These are reported to be very effective at catching out big carp. There are even some top anglers that still use this technique.
A similar technique can be done on present day rigs with long hairs. I can push the small piece of silicone tubing positioned on the shank, around the bend of the hook further until it causes the hair to leave the hook in the middle of the bend. This makes the point of the hook heavy, causing it to turn down into the flesh sooner. However, I would only do this when using a bottom bait.
The only downside to the long hair rig is the increased potential for tangles. Once a long hair wraps around the hook it can render the whole rig useless. However, there are a few ways to avoid long hair tangles during casting. You can thread a folded piece of rig foam onto the hair before placing a boilie on. This will trap and stiffen the section from the bend of the hook to the boilie and stop it from flying around during the cast. For this to technique to work properly, it’s best to do it on a hair rig that uses a silicone rubber tube on the shank of the hook. The silicone tube can be pushed up to help trap the hair more and make it secure for the cast. It won’t really work as well if you create a rig using a sliding rig ring on the hook shank.
Another way is to place a rig with long hairs in a PVA bag, or pulled inside a PVA dynamite stick, so that the whole hook is hidden inside and the hook bait can swing around outside the mesh where it cannot catch on anything to tangle up.
You can also just tie the long hair onto the shank using PVA tape, which is probably the best way to do it if you want to cast long distances. In deep water, or in weed, I would prefer to use foam threaded on the hair rig as I want the lead to hit the bottom before the PVA melts. If it loosens the hair before hitting the bottom, it could still wrap around the hook shank as it falls through the water.