I'm not a fan of neck carry. I have tried this a few times and I do recognize that in some situations it is perfect. For example, when sitting in a fishing boat, the sitting position tends to make a belt knife less accessible or have it poke you in the love handles. A neck knife, while you are sitting down, is much more accessible. At the same time, I find that I like this size of knife quite a bit. So for the majority of time I carry my necker as a pocket knife with occasional use as a necker. This shifts my priorities a bit with regards to ideal knife specs.
1) total length, between 5.5 - 6.5" no longer, otherwise it sticks out of my pocket
2) blade length - I think 2.5" is ideal, I like it to be at least 1" wide. I'll live with 3" but that starts cutting into the handle length
3) Handle length - 3.5" - 4" preferred. Although, with a set of scales and good ergos, a 3" handle can be quite adequate. JK's kit knife is remarkably hand friendly even though the handle is only 3". You grip it with three fingers and the pinky rests securly and comfortably on the end. You wouldn't have convinced me that this would be comfortable to use before I actually had in my hands. I prefer handles with slabs rather than plain or skeletal.
4) Blade shape - drop or clip. The key is to have a good belly. The belly maximizes cutting efficiency and in a small knife that is where the majority of cutting is going to occur. Another thing to consider is a robust tip. These little blades are often in a situation where you will use it for purposes that you shouldn't use a knife for. Lets face, if you carry a little pocket knife all the time, at some point you'll use it like a scredriver.....You know you will....You know its wrong....But you will do it.....Just a matter of time.....
5) Weight - I'm less concerned with this than handle comfort. I tend to like having handle slabs better than skeletal. The slabs increase ergos and decrease maintenance aspects. I love the look of wraps, but the fact of the matter is they get dirty and have to be re-wrapped. The handle slabs should be thing (1/8") to keep the knife profile slim.
Here are my neckers. I like all of them.
JK-kit knife
Breeden Kat
Jimmi Wade necker